Secure long training field (ltf)

ABSTRACT

This disclosure provides methods, devices and systems for generating a secure long training field (LTF). In some implementations, the secure LTF may include a randomized bit sequence that is difficult, if not impossible, to replicate by any device other than the transmitting device and the intended receiving device. For example, the transmitting device may use a block cipher or stream cipher to generate a pseudorandom bit sequence and may select a subset of bits of the pseudorandom bit sequence to be mapped to a sequence of modulation symbols representing an LTF symbol of the secure LTF. More specifically, each of the modulation symbols is mapped to a respective one of a number of subcarriers spanning a bandwidth of the secure LTF. The transmitting device may further transmit a physical layer convergence protocol (PLCP) protocol data unit (PPDU) that includes the secure LTF to the receiving device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation of U.S. patent applicationSer. No. 17/244,500 entitled, “SECURE LONG TRAINING FIELD (LTF)” filedApr. 29, 2021, which claims priority to U.S. Provisional PatentApplication No. 63/019,081 entitled “SECURE LONG TRAINING FIELD (LTF)”filed on May 1, 2020, and to U.S. Provisional Patent Application No.63/019,101 entitled “SECURE LONG TRAINING FIELD (LTF)” filed on May 1,2020, to U.S. Provisional Patent Application No. 63/076,181 entitled“SECURE LONG TRAINING FIELD (LTF)” and filed on Sep. 9, 2020, each ofwhich are assigned to the assignee hereof, and each of which areexpressly incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates generally to wireless communication, and morespecifically, to secure long training fields (LTFs) for wirelesscommunications.

DESCRIPTION OF THE RELATED TECHNOLOGY

A wireless local area network (WLAN) may be formed by one or more accesspoints (APs) that provide a shared wireless communication medium for useby a number of client devices also referred to as stations (STAs). Thebasic building block of a WLAN conforming to the Institute of Electricaland Electronics Engineers (IEEE) 802.11 family of standards is a BasicService Set (BSS), which is managed by an AP. Each BSS is identified bya Basic Service Set Identifier (BSSID) that is advertised by the AP. AnAP periodically broadcasts beacon frames to enable any STAs withinwireless range of the AP to establish or maintain a communication linkwith the WLAN.

The IEEE 802.11 family of standards define a packet format, to be usedfor wireless communication, which includes one or more long trainingfields (LTFs). LTFs are generally used for channel estimation purposes.For example, a transmitting device may transmit a known pattern ofsymbols, in an LTF, to a receiving device. The receiving device may useits knowledge of the symbol pattern in the received LTF to estimate howwireless communications propagate through a wireless channel between thetransmitting device and the receiving device. Unlike data fields, LTFsdo not carry any useful information or user-specific data. Thus, inaccordance with existing versions of the IEEE 802.11 standard, LTFsymbols are transmitted with very little or no security. However, recentamendments to the IEEE 802.11 standard (such as 802.11az) have expandedthe uses for LTFs in ways which may be subject to attack. It istherefore desirable to provide greater security for LTFs used in somewireless communications.

SUMMARY

The systems, methods and devices of this disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

One innovative aspect of the subject matter described in this disclosurecan be implemented as a method of wireless communication. The method maybe performed by a wireless communication device, and may includegenerating a pseudorandom bit sequence; selecting a first subset of bitsof the pseudorandom bit sequence based on a number (N) of subcarriersassociated with a long training field (LTF) of a physical (PHY) layerconvergence protocol (PLCP) protocol data unit (PPDU), where a number ofbits in the first subset of bits is greater than N; mapping values ofthe first subset of bits to a sequence of first modulation symbolsrepresenting a first LTF symbol of the LTF, where each of the firstmodulation symbols is modulated on a respective one of the Nsubcarriers; and transmitting the PPDU, including the LTF, to areceiving device.

In some implementations, the pseudorandom bit sequence may be generatedin a PHY layer of the wireless communication device. In someimplementations, the pseudorandom bit sequence may be generated based onan output of an advanced encryption standard (AES) block cipher. In someaspects, the generating of the pseudorandom bit sequence may includegenerating a set of secure bits in a media access control (MAC) layer ofthe wireless communication device and initializing the block cipher inthe PHY layer of the wireless communication device based on the set ofsecure bits from the MAC layer.

In some implementations, the mapping of the values of the first subsetof bits to the sequence of first modulation symbols may be performed inaccordance with a quadrature amplitude modulation (QAM) scheme. In someaspects, each of the first modulation symbols may be a 64-QAM symbol. Insome implementations, the first subset of bits may be selected from aportion of the pseudorandom bit sequence that does not include anyrepetitions.

In some implementations, the method may further include mapping thesequence of first modulation symbols to a number (M) of spatial streamsand applying M sets of first phase rotations to the sequence of firstmodulation symbols mapped to the M spatial streams, respectively, whereeach set of the M sets of first phase rotations is different than theremaining M−1 sets of first phase rotations. In some aspects, the methodmay further include generating the M sets of first phase rotations basedon a pseudorandom output of a linear feedback shift register (LFSR).

In some implementations, the method may further include selecting asecond subset of bits of the pseudorandom bit sequence, where the secondsubset of bits is different than the first subset of bits; mappingvalues of the second subset of bits to a sequence of second modulationsymbols representing a second LTF symbol of the LTF, where each of thesecond modulation symbols is modulated on a respective one of the Nsubcarriers; mapping the sequence of second modulation symbols to the Mspatial streams; and applying the M sets of first phase rotations to thesequence of second modulation symbols mapped to the M spatial streams,respectively. In some aspects, the second subset of bits may be selectedfrom a portion of the pseudorandom bit sequence that does not includeany repetitions or bits from the first subset.

In some implementations, the method may further include mapping thevalues of the first subset of bits to a sequence of second modulationsymbols representing a second LTF symbol of the LTF, where each of thesecond modulation symbols is modulated on a respective one of the Nsubcarriers; mapping the sequence of second modulation symbols to the Mspatial streams; and applying M sets of second phase rotations to thesequence of second modulation symbols mapped to the M spatial streams,respectively, where each set of the M sets of second phase rotations isdifferent than the remaining M−1 sets of second phase rotations anddifferent than the M sets of first phase rotations.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a wireless communication device. Insome implementations, the wireless communication device may include atleast one modem, at least one processor communicatively coupled with theat least one modem, and at least one memory communicatively coupled withthe at least one processor and storing processor-readable code. In someimplementations, execution of the processor-readable code by the atleast one processor causes the wireless communication device to performoperations including generating a pseudorandom bit sequence; selecting afirst subset of bits of the pseudorandom bit sequence based on a number(N) of subcarriers associated with an LTF of a PPDU, where a number ofbits in the first subset of bits is greater than N; mapping values ofthe first subset of bits to a sequence of first modulation symbolsrepresenting a first LTF symbol of the LTF, where each of the firstmodulation symbols is modulated on a respective one of the Nsubcarriers; and transmitting the PPDU, including the LTF, to areceiving device.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented as a method of wireless communication. Themethod may be performed by a wireless communication device, and mayinclude generating a pseudorandom bit sequence; receiving a PPDU, over awireless channel, from a transmitting device; recovering a sequence offirst modulation symbols from an LTF of the received PPDU, where thesequence of first modulation symbols represents a first LTF symbol ofthe LTF; demodulating each of the first modulation symbols from arespective one of a number (N) of subcarriers associated with the LTF,where the demodulation of the first modulation symbols produces a firstsubset of bits representing the first LTF symbol; and estimating thewireless channel based on the first subset of bits and the pseudorandombit sequence.

In some implementations, the pseudorandom bit sequence may be generatedin a PHY layer of the wireless communication device. In someimplementations, the pseudorandom bit sequence may be generated based onan output of an AES block cipher. In some aspects, the generating of thepseudorandom bit sequence may include generating a set of secure bits ina MAC layer of the wireless communication device and initializing theAES block cipher block in the PHY layer of the wireless communicationdevice based on the set of secure bits from the MAC layer.

In some implementations, each of the first modulation symbols may bedemodulated in accordance with a QAM scheme. In some aspects, each ofthe first modulation symbols may be a 64-QAM symbol.

In some implementations, the PPDU may be received on a number (M) ofspatial streams and the recovering of the sequence of first modulationsymbols may include applying M sets of first phase rotations to the Mspatial streams, respectively, where each set of the M sets of firstphase rotations is different than the remaining M−1 sets of first phaserotations. In some aspects, the method may further include generatingthe M sets of first phase rotations based on a pseudorandom output of anLFSR.

In some implementations, the method may further include recovering asequence of second modulation symbols from the LTF of the received PPDU,where the sequence of second modulation symbols represents a second LTFsymbol of the LTF; and demodulating each of the second modulationsymbols from a respective one of the N subcarriers, where thedemodulation of the second modulation symbols produces a second subsetof bits representing the second LTF symbol, and where the wirelesschannel estimate is based on the first subset of bits, the second subsetof bits, and the pseudorandom bit sequence.

In some implementations, the recovering of the sequence of secondmodulation symbols may include applying the M sets of first phaserotations to the M spatial streams, respectively. In some otherimplementations, the recovering of the sequence of second modulationsymbols may include applying the M sets of second phase rotations to theM spatial streams, respectively, where each of the M sets of secondphase rotations is different than the remaining M−1 sets of second phaserotations and different than the M sets of first phase rotations.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a wireless communication device. Insome implementations, the wireless communication device may include atleast one modem, at least one processor communicatively coupled with theat least one modem, and at least one memory communicatively coupled withthe at least one processor and storing processor-readable code. In someimplementations, execution of the processor-readable code by the atleast one processor causes the wireless communication device to performoperations including generating a pseudorandom bit sequence; receiving aPPDU, over a wireless channel, from a transmitting device; recovering asequence of first modulation symbols from an LTF of the received PPDU,where the sequence of first modulation symbols represents a first LTFsymbol of the LTF; demodulating each of the first modulation symbolsfrom a respective one of a number (N) of subcarriers associated with theLTF, where the demodulation of the first modulation symbols produces afirst subset of bits representing the first LTF symbol; and estimatingthe wireless channel based on the first subset of bits and thepseudorandom bit sequence

BRIEF DESCRIPTION OF THE DRAWINGS

Details of one or more implementations of the subject matter describedin this disclosure are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

FIG. 1 shows a pictorial diagram of an example wireless communicationnetwork.

FIG. 2A shows an example protocol data unit (PDU) usable forcommunications between an access point (AP) and one or more stations(STAs).

FIG. 2B shows an example field in the PDU of FIG. 2A.

FIG. 3A shows an example PHY layer convergence protocol (PLCP) protocoldata unit (PPDU) usable for communications between an AP and one or moreSTAs.

FIG. 3B shows another example PPDU usable for communications between anAP and one or more STAs.

FIG. 4 shows an example PHY protocol data unit (PPDU) usable forcommunications between an AP and one or more STAs.

FIG. 5 shows a block diagram of an example wireless communicationdevice.

FIG. 6A shows a block diagram of an example access point (AP).

FIG. 6B shows a block diagram of an example station (STA).

FIG. 7 shows a timing diagram illustrating an example process forperforming a ranging operation.

FIG. 8A shows a frequency diagram of an example long training field(LTF) sequence usable for communications between wireless communicationdevices.

FIG. 8B shows a timing diagram of an example LTF symbol usable forcommunications between wireless communication devices.

FIGS. 9A and 9B show block diagrams of an example transmit (TX)processing chain of a wireless communication device according to someimplementations.

FIG. 10A shows a frequency diagram of an example LTF symbol prior tointercarrier interference (ICI) injection according to someimplementations.

FIG. 10B shows a frequency diagram of an example LTF symbol after ICIinjection according to some implementations.

FIG. 11A shows a frequency diagram of an example phase ramp according tosome implementations.

FIG. 11B shows a frequency diagram of an example phase ramp according tosome implementations.

FIGS. 12A and 12B show block diagrams of an example receive (RX)processing chain of a wireless communication device according to someimplementations.

FIG. 13A shows a flowchart illustrating an example process for wirelesscommunication that supports secure LTFs according to someimplementations.

FIG. 13B shows a flowchart illustrating an example process for wirelesscommunication that supports secure LTFs according to someimplementations.

FIG. 13C shows a flowchart illustrating an example process 1320 forwireless communication that supports secure LTFs according to someimplementations.

FIG. 14A shows a flowchart illustrating an example process for wirelesscommunication that supports secure LTFs according to someimplementations.

FIG. 14B shows a flowchart illustrating an example process for wirelesscommunication that supports secure LTFs according to someimplementations.

FIG. 15 shows a block diagram of an example wireless communicationdevice according to some implementations.

FIG. 16 shows a block diagram of an example wireless communicationdevice according to some implementations.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The following description is directed to some particular implementationsfor the purposes of describing innovative aspects of this disclosure.However, a person having ordinary skill in the art will readilyrecognize that the teachings herein can be applied in a multitude ofdifferent ways. The described implementations can be implemented in anydevice, system or network that is capable of transmitting and receivingradio frequency (RF) signals according to one or more of the Instituteof Electrical and Electronics Engineers (IEEE) 802.11 standards, theIEEE 802.15 standards, the Bluetooth® standards as defined by theBluetooth Special Interest Group (SIG), or the Long Term Evolution(LTE), 3G, 4G or 5G (New Radio (NR)) standards promulgated by the 3rdGeneration Partnership Project (3GPP), among others. The describedimplementations can be implemented in any device, system or network thatis capable of transmitting and receiving RF signals according to one ormore of the following technologies or techniques: code division multipleaccess (CDMA), time division multiple access (TDMA), frequency divisionmultiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA(SC-FDMA), single-user (SU) multiple-input multiple-output (MIMO) andmulti-user (MU) MIMO. The described implementations also can beimplemented using other wireless communication protocols or RF signalssuitable for use in one or more of a wireless personal area network(WPAN), a wireless local area network (WLAN), a wireless wide areanetwork (WWAN), or an internet of things (IOT) network.

Various aspects relate generally to long training fields (LTFs) used inwireless communications, and more particularly, to generating a secureLTF that is difficult to decode or replicate by observing only a portionof the LTF. In some aspects, the secure LTF may include a randomized bitsequence that is difficult, if not impossible, to replicate by anydevice other than the transmitting device and the intended receivingdevice (using a secure key previously shared over a secure wirelesslink). For example, the transmitting device may use a block or a streamcipher to generate a pseudorandom bit sequence and may select a subsetof bits of the pseudorandom bit sequence to be mapped to a sequence ofmodulation symbols (also referred to herein as an “LTF sequence”)representing an LTF symbol of the secure LTF. More specifically, each ofthe modulation symbols is mapped to a respective one of a number ofsubcarriers spanning a bandwidth of the secure LTF. The transmittingdevice may further transmit a physical layer convergence protocol (PLCP)protocol data unit (PPDU) that includes the secure LTF to the receivingdevice.

Particular implementations of the subject matter described in thisdisclosure can be implemented to realize one or more of the followingpotential advantages. In some implementations, the described techniquescan be used to improve the security of LTFs used in wirelesscommunications. For example, LTF sequences conforming to existingversions of the IEEE 802.11 standard are encoded or modulated based ondeterministic functions. As a result, an attacker (or unintendedreceiving device) may receive a portion of an LTF sequence and determineor predict the remainder of the LTF sequence based on the receivedportion. A sophisticated attacker may even copy or spoof the LTFsequence before the transmitting device has finished transmitting theoriginal LTF sequence to the receiving device. For example, the attackermay transmit the spoofed LTF sequence to the receiving device to causeerrors in channel or timing measurements by the receiving device. Byrandomizing the modulation symbols associated with individual LTFsequences, aspects of the present disclosure may prevent orsubstantially delay such attacks on LTF sequences long enough to renderthe attacks ineffective.

FIG. 1 shows a block diagram of an example wireless communicationnetwork 100. According to some aspects, the wireless communicationnetwork 100 can be an example of a wireless local area network (WLAN)such as a Wi-Fi network (and will hereinafter be referred to as WLAN100). For example, the WLAN 100 can be a network implementing at leastone of the IEEE 802.11 family of wireless communication protocolstandards (such as that defined by the IEEE 802.11-2020 specification oramendments thereof including, but not limited to, 802.11ah, 802.11ad,802.11ay, 802.11ax, 802.11az, 802.11ba and 802.11be). The WLAN 100 mayinclude numerous wireless communication devices such as an access point(AP) 102 and multiple stations (STAs) 104. While only one AP 102 isshown, the WLAN network 100 also can include multiple APs 102.

Each of the STAs 104 also may be referred to as a mobile station (MS), amobile device, a mobile handset, a wireless handset, an access terminal(AT), a user equipment (UE), a subscriber station (SS), or a subscriberunit, among other possibilities. The STAs 104 may represent variousdevices such as mobile phones, personal digital assistant (PDAs), otherhandheld devices, netbooks, notebook computers, tablet computers,laptops, display devices (for example, TVs, computer monitors,navigation systems, among others), music or other audio or stereodevices, remote control devices (“remotes”), printers, kitchen or otherhousehold appliances, key fobs (for example, for passive keyless entryand start (PKES) systems), among other possibilities.

A single AP 102 and an associated set of STAs 104 may be referred to asa basic service set (BSS), which is managed by the respective AP 102.FIG. 1 additionally shows an example coverage area 108 of the AP 102,which may represent a basic service area (BSA) of the WLAN 100. The BSSmay be identified to users by a service set identifier (SSID), as wellas to other devices by a basic service set identifier (BSSID), which maybe a medium access control (MAC) address of the AP 102. The AP 102periodically broadcasts beacon frames (“beacons”) including the BSSID toenable any STAs 104 within wireless range of the AP 102 to “associate”or re-associate with the AP 102 to establish a respective communicationlink 106 (hereinafter also referred to as a “Wi-Fi link”), or tomaintain a communication link 106, with the AP 102. For example, thebeacons can include an identification of a primary channel used by therespective AP 102 as well as a timing synchronization function forestablishing or maintaining timing synchronization with the AP 102. TheAP 102 may provide access to external networks to various STAs 104 inthe WLAN via respective communication links 106.

To establish a communication link 106 with an AP 102, each of the STAs104 is configured to perform passive or active scanning operations(“scans”) on frequency channels in one or more frequency bands (forexample, the 2.4 GHz, 5 GHz, 6 GHz or 60 GHz bands). To perform passivescanning, a STA 104 listens for beacons, which are transmitted byrespective APs 102 at a periodic time interval referred to as the targetbeacon transmission time (TBTT) (measured in time units (TUs) where oneTU may be equal to 1024 microseconds (μs)). To perform active scanning,a STA 104 generates and sequentially transmits probe requests on eachchannel to be scanned and listens for probe responses from APs 102. EachSTA 104 may be configured to identify or select an AP 102 with which toassociate based on the scanning information obtained through the passiveor active scans, and to perform authentication and associationoperations to establish a communication link 106 with the selected AP102. The AP 102 assigns an association identifier (AID) to the STA 104at the culmination of the association operations, which the AP 102 usesto track the STA 104.

As a result of the increasing ubiquity of wireless networks, a STA 104may have the opportunity to select one of many BSSs within range of theSTA or to select among multiple APs 102 that together form an extendedservice set (ESS) including multiple connected BSSs. An extended networkstation associated with the WLAN 100 may be connected to a wired orwireless distribution system that may allow multiple APs 102 to beconnected in such an ESS. As such, a STA 104 can be covered by more thanone AP 102 and can associate with different APs 102 at different timesfor different transmissions. Additionally, after association with an AP102, a STA 104 also may be configured to periodically scan itssurroundings to find a more suitable AP 102 with which to associate. Forexample, a STA 104 that is moving relative to its associated AP 102 mayperform a “roaming” scan to find another AP 102 having more desirablenetwork characteristics such as a greater received signal strengthindicator (RSSI) or a reduced traffic load.

In some cases, STAs 104 may form networks without APs 102 or otherequipment other than the STAs 104 themselves. One example of such anetwork is an ad hoc network (or wireless ad hoc network). Ad hocnetworks may alternatively be referred to as mesh networks orpeer-to-peer (P2P) networks. In some cases, ad hoc networks may beimplemented within a larger wireless network such as the WLAN 100. Insuch implementations, while the STAs 104 may be capable of communicatingwith each other through the AP 102 using communication links 106, STAs104 also can communicate directly with each other via direct wirelesslinks 110. Additionally, two STAs 104 may communicate via a directcommunication link 110 regardless of whether both STAs 104 areassociated with and served by the same AP 102. In such an ad hoc system,one or more of the STAs 104 may assume the role filled by the AP 102 ina BSS. Such a STA 104 may be referred to as a group owner (GO) and maycoordinate transmissions within the ad hoc network. Examples of directwireless links 110 include Wi-Fi Direct connections, connectionsestablished by using a Wi-Fi Tunneled Direct Link Setup (TDLS) link, andother P2P group connections.

The APs 102 and STAs 104 may function and communicate (via therespective communication links 106) according to the IEEE 802.11 familyof wireless communication protocol standards (such as that defined bythe IEEE 802.11-2016 specification or amendments thereof including, butnot limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az,802.11ba and 802.11be). These standards define the WLAN radio andbaseband protocols for the PHY and medium access control (MAC) layers.The APs 102 and STAs 104 transmit and receive wireless communications(hereinafter also referred to as “Wi-Fi communications”) to and from oneanother in the form of physical layer convergence protocol (PLCP)protocol data units (PPDUs). The APs 102 and STAs 104 in the WLAN 100may transmit PPDUs over an unlicensed spectrum, which may be a portionof spectrum that includes frequency bands traditionally used by Wi-Fitechnology, such as the 2.4 GHz band, the 5 GHz band, the 60 GHz band,the 3.6 GHz band, and the 700 MHz band. Some implementations of the APs102 and STAs 104 described herein also may communicate in otherfrequency bands, such as the 6 GHz band, which may support both licensedand unlicensed communications. The APs 102 and STAs 104 also can beconfigured to communicate over other frequency bands such as sharedlicensed frequency bands, where multiple operators may have a license tooperate in the same or overlapping frequency band or bands.

Each of the frequency bands may include multiple sub-bands or frequencychannels. For example, PPDUs conforming to the IEEE 802.11n, 802.11ac,802.11ax and 802.11be standard amendments may be transmitted over the2.4, 5 GHz or 6 GHz bands, each of which is divided into multiple 20 MHzchannels. As such, these PPDUs are transmitted over a physical channelhaving a minimum bandwidth of 20 MHz, but larger channels can be formedthrough channel bonding. For example, PPDUs may be transmitted overphysical channels having bandwidths of 40 MHz, 80 MHz, 160 or 320 MHz bybonding together multiple 20 MHz channels.

Each PPDU is a composite structure that includes a PHY preamble and apayload in the form of a PHY service data unit (PSDU). The informationprovided in the preamble may be used by a receiving device to decode thesubsequent data in the PSDU. In instances in which PPDUs are transmittedover a bonded channel, the preamble fields may be duplicated andtransmitted in each of the multiple component channels. The PHY preamblemay include both a legacy portion (or “legacy preamble”) and anon-legacy portion (or “non-legacy preamble”). The legacy preamble maybe used for packet detection, automatic gain control and channelestimation, among other uses. The legacy preamble also may generally beused to maintain compatibility with legacy devices. The format of,coding of, and information provided in the non-legacy portion of thepreamble is based on the particular IEEE 802.11 protocol to be used totransmit the payload.

FIG. 2A shows an example protocol data unit (PDU) 200 usable forwireless communication between an AP 102 and one or more STAs 104. Forexample, the PDU 200 can be configured as a PPDU. As shown, the PDU 200includes a PHY preamble 202 and a PHY payload 204. For example, thepreamble 202 may include a legacy portion that itself includes a legacyshort training field (L-STF) 206, which may consist of BPSK symbols, alegacy long training field (L-LTF) 208, which may consist of BPSKsymbols, and a legacy signal field (L-SIG) 210, which may consist ofBPSK symbols. The legacy portion of the preamble 202 may be configuredaccording to the IEEE 802.11a wireless communication protocol standard.The preamble 202 may also include a non-legacy portion including one ormore non-legacy fields 212, for example, conforming to an IEEE wirelesscommunication protocol such as the IEEE 802.11ac, 802.11ax, 802.11be orlater wireless communication protocol protocols.

The L-STF 206 generally enables a receiving device to perform automaticgain control (AGC) and coarse timing and frequency estimation. The L-LTF208 generally enables a receiving device to perform fine timing andfrequency estimation and also to perform an initial estimate of thewireless channel. The L-SIG 210 generally enables a receiving device todetermine a duration of the PDU and to use the determined duration toavoid transmitting on top of the PDU. For example, the L-STF 206, theL-LTF 208 and the L-SIG 210 may be modulated according to a binary phaseshift keying (BPSK) modulation scheme. The payload 204 may be modulatedaccording to a BPSK modulation scheme, a quadrature BPSK (Q-BPSK)modulation scheme, a quadrature amplitude modulation (QAM) modulationscheme, or another appropriate modulation scheme. The payload 204 mayinclude a PSDU including a data field (DATA) 214 that, in turn, maycarry higher layer data, for example, in the form of medium accesscontrol (MAC) protocol data units (MPDUs) or an aggregated MPDU(A-MPDU).

FIG. 2B shows an example L-SIG 210 in the PDU 200 of FIG. 2A. The L-SIG210 includes a data rate field 222, a reserved bit 224, a length field226, a parity bit 228, and a tail field 230. The data rate field 222indicates a data rate (note that the data rate indicated in the datarate field 212 may not be the actual data rate of the data carried inthe payload 204). The length field 226 indicates a length of the packetin units of, for example, symbols or bytes. The parity bit 228 may beused to detect bit errors. The tail field 230 includes tail bits thatmay be used by the receiving device to terminate operation of a decoder(for example, a Viterbi decoder). The receiving device may utilize thedata rate and the length indicated in the data rate field 222 and thelength field 226 to determine a duration of the packet in units of, forexample, microseconds s) or other time units.

FIG. 3A shows an example PPDU 300 usable for wireless communicationbetween an AP and one or more STAs. The PPDU 300 may be used for SU,OFDMA or MU-MIMO transmissions. The PPDU 300 may be formatted as a HighEfficiency (HE) WLAN PPDU in accordance with the IEEE 802.11ax amendmentto the IEEE 802.11 wireless communication protocol standard. The PPDU300 includes a PHY preamble including a legacy portion 302 and anon-legacy portion 304. The PPDU 300 may further include a PHY payload306 after the preamble, for example, in the form of a PSDU including adata field 324.

The legacy portion 302 of the preamble includes an L-STF 308, an L-LTF310, and an L-SIG 312. The non-legacy portion 304 includes a repetitionof L-SIG (RL-SIG) 314, a first HE signal field (HE-SIG-A) 316, an HEshort training field (HE-STF) 320, and one or more HE long trainingfields (or symbols) (HE-LTFs) 322. For OFDMA or MU-MIMO communications,the second portion 304 further includes a second HE signal field(HE-SIG-B) 318 encoded separately from HE-SIG-A 316. Like the L-STF 308,L-LTF 310, and L-SIG 312, the information in RL-SIG 314 and HE-SIG-A 316may be duplicated and transmitted in each of the component 20 MHzchannels in instances involving the use of a bonded channel. Incontrast, the content in HE-SIG-B 318 may be unique to each 20 MHzchannel and target specific STAs 104.

RL-SIG 314 may indicate to HE-compatible STAs 104 that the PPDU 300 isan HE PPDU. An AP 102 may use HE-SIG-A 316 to identify and informmultiple STAs 104 that the AP has scheduled UL or DL resources for them.For example, HE-SIG-A 316 may include a resource allocation subfieldthat indicates resource allocations for the identified STAs 104.HE-SIG-A 316 may be decoded by each HE-compatible STA 104 served by theAP 102. For MU transmissions, HE-SIG-A 316 further includes informationusable by each identified STA 104 to decode an associated HE-SIG-B 318.For example, HE-SIG-A 316 may indicate the frame format, includinglocations and lengths of HE-SIG-Bs 318, available channel bandwidths andmodulation and coding schemes (MCSs), among other examples. HE-SIG-A 316also may include HE WLAN signaling information usable by STAs 104 otherthan the identified STAs 104.

HE-SIG-B 318 may carry STA-specific scheduling information such as, forexample, STA-specific (or “user-specific”) MCS values and STA-specificRU allocation information. In the context of DL MU-OFDMA, suchinformation enables the respective STAs 104 to identify and decodecorresponding resource units (RUs) in the associated data field 324.Each HE-SIG-B 318 includes a common field and at least one STA-specificfield. The common field can indicate RU allocations to multiple STAs 104including RU assignments in the frequency domain, indicate which RUs areallocated for MU-MIMO transmissions and which RUs correspond to MU-OFDMAtransmissions, and the number of users in allocations, among otherexamples. The common field may be encoded with common bits, CRC bits,and tail bits. The user-specific fields are assigned to particular STAs104 and may be used to schedule specific RUs and to indicate thescheduling to other WLAN devices. Each user-specific field may includemultiple user block fields. Each user block field may include two userfields that contain information for two respective STAs to decode theirrespective RU payloads in data field 324.

FIG. 3B shows another example PPDU 350 usable for wireless communicationbetween an AP and one or more STAs. The PPDU 350 may be used for SU,OFDMA or MU-MIMO transmissions. The PPDU 350 may be formatted as anExtreme High Throughput (EHT) WLAN PPDU in accordance with the IEEE802.11be amendment to the IEEE 802.11 wireless communication protocolstandard, or may be formatted as a PPDU conforming to any later(post-EHT) version of a new wireless communication protocol conformingto a future IEEE 802.11 wireless communication protocol standard orother wireless communication standard. The PPDU 350 includes a PHYpreamble including a legacy portion 352 and a non-legacy portion 354.The PPDU 350 may further include a PHY payload 356 after the preamble,for example, in the form of a PSDU including a data field 374.

The legacy portion 352 of the preamble includes an L-STF 358, an L-LTF360, and an L-SIG 362. The non-legacy portion 354 of the preambleincludes an RL-SIG 364 and multiple wireless communication protocolversion-dependent signal fields after RL-SIG 364. For example, thenon-legacy portion 354 may include a universal signal field 366(referred to herein as “U-SIG 366”) and an EHT signal field 368(referred to herein as “EHT-SIG 368”). One or both of U-SIG 366 andEHT-SIG 368 may be structured as, and carry version-dependentinformation for, other wireless communication protocol versions beyondEHT. The non-legacy portion 354 further includes an additional shorttraining field 370 (referred to herein as “EHT-STF 370,” although it maybe structured as, and carry version-dependent information for, otherwireless communication protocol versions beyond EHT) and one or moreadditional long training fields 372 (referred to herein as “EHT-LTFs372,” although they may be structured as, and carry version-dependentinformation for, other wireless communication protocol versions beyondEHT). Like L-STF 358, L-LTF 360, and L-SIG 362, the information in U-SIG366 and EHT-SIG 368 may be duplicated and transmitted in each of thecomponent 20 MHz channels in instances involving the use of a bondedchannel. In some implementations, EHT-SIG 368 may additionally oralternatively carry information in one or more non-primary 20 MHzchannels that is different than the information carried in the primary20 MHz channel.

EHT-SIG 368 may include one or more jointly encoded symbols and may beencoded in a different block from the block in which U-SIG 366 isencoded. EHT-SIG 368 may be used by an AP to identify and informmultiple STAs 104 that the AP has scheduled UL or DL resources for them.EHT-SIG 368 may be decoded by each compatible STA 104 served by the AP102. EHT-SIG 368 may generally be used by a receiving device tointerpret bits in the data field 374. For example, EHT-SIG 368 mayinclude RU allocation information, spatial stream configurationinformation, and per-user signaling information such as MCSs, amongother examples. EHT-SIG 368 may further include a cyclic redundancycheck (CRC) (for example, four bits) and a tail (for example, 6 bits)that may be used for binary convolutional code (BCC). In someimplementations, EHT-SIG 368 may include one or more code blocks thateach include a CRC and a tail. In some aspects, each of the code blocksmay be encoded separately.

EHT-SIG 368 may carry STA-specific scheduling information such as, forexample, user-specific MCS values and user-specific RU allocationinformation. EHT-SIG 368 may generally be used by a receiving device tointerpret bits in the data field 374. In the context of DL MU-OFDMA,such information enables the respective STAs 104 to identify and decodecorresponding RUs in the associated data field 374. Each EHT-SIG 368 mayinclude a common field and at least one user-specific field. The commonfield can indicate RU distributions to multiple STAs 104, indicate theRU assignments in the frequency domain, indicate which RUs are allocatedfor MU-MIMO transmissions and which RUs correspond to MU-OFDMAtransmissions, and the number of users in allocations, among otherexamples. The common field may be encoded with common bits, CRC bits,and tail bits. The user-specific fields are assigned to particular STAs104 and may be used to schedule specific RUs and to indicate thescheduling to other WLAN devices. Each user-specific field may includemultiple user block fields. Each user block field may include, forexample, two user fields that contain information for two respectiveSTAs to decode their respective RU payloads.

The presence of RL-SIG 364 and U-SIG 366 may indicate to EHT- or laterversion-compliant STAs 104 that the PPDU 350 is an EHT PPDU or a PPDUconforming to any later (post-EHT) version of a new wirelesscommunication protocol conforming to a future IEEE 802.11 wirelesscommunication protocol standard. For example, U-SIG 366 may be used by areceiving device to interpret bits in one or more of EHT-SIG 368 or thedata field 374.

FIG. 4 shows an example PPDU 400 usable for communications between an AP102 and one or more STAs 104. As described above, each PPDU 400 includesa PHY preamble 402 and a PSDU 404. Each PSDU 404 may represent (or“carry”) one or more MAC protocol data units (MPDUs) 416. For example,each PSDU 404 may carry an aggregated MPDU (A-MPDU) 406 that includes anaggregation of multiple A-MPDU subframes 408. Each A-MPDU subframe 406may include an MPDU frame 410 that includes a MAC delimiter 412 and aMAC header 414 prior to the accompanying MPDU 416, which comprises thedata portion (“payload” or “frame body”) of the MPDU frame 410. EachMPDU frame 410 may also include a frame check sequence (FCS) field 418for error detection (for example, the FCS field may include a cyclicredundancy check (CRC)) and padding bits 420. The MPDU 416 may carry oneor more MAC service data units (MSDUs) 426. For example, the MPDU 416may carry an aggregated MSDU (A-MSDU) 422 including multiple A-MSDUsubframes 424. Each A-MSDU subframe 424 contains a corresponding MSDU430 preceded by a subframe header 428 and in some cases followed bypadding bits 432.

Referring back to the MPDU frame 410, the MAC delimiter 412 may serve asa marker of the start of the associated MPDU 416 and indicate the lengthof the associated MPDU 416. The MAC header 414 may include multiplefields containing information that defines or indicates characteristicsor attributes of data encapsulated within the frame body 416. The MACheader 414 includes a duration field indicating a duration extendingfrom the end of the PPDU until at least the end of an acknowledgment(ACK) or Block ACK (BA) of the PPDU that is to be transmitted by thereceiving wireless communication device. The use of the duration fieldserves to reserve the wireless medium for the indicated duration, andenables the receiving device to establish its network allocation vector(NAV). The MAC header 414 also includes one or more fields indicatingaddresses for the data encapsulated within the frame body 416. Forexample, the MAC header 414 may include a combination of a sourceaddress, a transmitter address, a receiver address or a destinationaddress. The MAC header 414 may further include a frame control fieldcontaining control information. The frame control field may specify aframe type, for example, a data frame, a control frame, or a managementframe.

FIG. 5 shows a block diagram of an example wireless communication device500. In some implementations, the wireless communication device 500 canbe an example of a device for use in a STA such as one of the STAs 104described with reference to FIG. 1 . In some implementations, thewireless communication device 500 can be an example of a device for usein an AP such as the AP 102 described with reference to FIG. 1 . Thewireless communication device 500 is capable of transmitting (oroutputting for transmission) and receiving wireless communications (forexample, in the form of wireless packets). For example, the wirelesscommunication device can be configured to transmit and receive packetsin the form of physical layer convergence protocol (PLCP) protocol dataunits (PPDUs) and medium access control (MAC) protocol data units(MPDUs) conforming to an IEEE 802.11 wireless communication protocolstandard, such as that defined by the IEEE 802.11-2016 specification oramendments thereof including, but not limited to, 802.11ah, 802.11ad,802.11ay, 802.11ax, 802.11az, 802.11ba and 802.11be.

The wireless communication device 500 can be, or can include, a chip,system on chip (SoC), chipset, package or device that includes one ormore modems 502, for example, a Wi-Fi (IEEE 802.11 compliant) modem. Insome implementations, the one or more modems 502 (collectively “themodem 502”) additionally include a WWAN modem (for example, a 3GPP 4GLTE or 5G compliant modem). In some implementations, the wirelesscommunication device 500 also includes one or more radios 504(collectively “the radio 504”). In some implementations, the wirelesscommunication device 506 further includes one or more processors,processing blocks or processing elements 506 (collectively “theprocessor 506”) and one or more memory blocks or elements 508(collectively “the memory 508”).

The modem 502 can include an intelligent hardware block or device suchas, for example, an application-specific integrated circuit (ASIC) amongother possibilities. The modem 502 is generally configured to implementa PHY layer. For example, the modem 502 is configured to modulatepackets and to output the modulated packets to the radio 504 fortransmission over the wireless medium. The modem 502 is similarlyconfigured to obtain modulated packets received by the radio 504 and todemodulate the packets to provide demodulated packets. In addition to amodulator and a demodulator, the modem 502 may further include digitalsignal processing (DSP) circuitry, automatic gain control (AGC), acoder, a decoder, a multiplexer and a demultiplexer. For example, whilein a transmission mode, data obtained from the processor 506 is providedto a coder, which encodes the data to provide encoded bits. The encodedbits are then mapped to points in a modulation constellation (using aselected MCS) to provide modulated symbols. The modulated symbols maythen be mapped to a number N_(SS) of spatial streams or a number N_(STS)of space-time streams. The modulated symbols in the respective spatialor space-time streams may then be multiplexed, transformed via aninverse fast Fourier transform (IFFT) block, and subsequently providedto the DSP circuitry for Tx windowing and filtering. The digital signalsmay then be provided to a digital-to-analog converter (DAC). Theresultant analog signals may then be provided to a frequencyupconverter, and ultimately, the radio 504. In implementations involvingbeamforming, the modulated symbols in the respective spatial streams areprecoded via a steering matrix prior to their provision to the IFFTblock.

While in a reception mode, digital signals received from the radio 504are provided to the DSP circuitry, which is configured to acquire areceived signal, for example, by detecting the presence of the signaland estimating the initial timing and frequency offsets. The DSPcircuitry is further configured to digitally condition the digitalsignals, for example, using channel (narrowband) filtering, analogimpairment conditioning (such as correcting for I/Q imbalance), andapplying digital gain to ultimately obtain a narrowband signal. Theoutput of the DSP circuitry may then be fed to the AGC, which isconfigured to use information extracted from the digital signals, forexample, in one or more received training fields, to determine anappropriate gain. The output of the DSP circuitry also is coupled withthe demodulator, which is configured to extract modulated symbols fromthe signal and, for example, compute the logarithm likelihood ratios(LLRs) for each bit position of each subcarrier in each spatial stream.The demodulator is coupled with the decoder, which may be configured toprocess the LLRs to provide decoded bits. The decoded bits from all ofthe spatial streams are then fed to the demultiplexer fordemultiplexing. The demultiplexed bits may then be descrambled andprovided to the MAC layer (the processor 506) for processing, evaluationor interpretation.

The radio 504 generally includes at least one radio frequency (RF)transmitter (or “transmitter chain”) and at least one RF receiver (or“receiver chain”), which may be combined into one or more transceivers.For example, the RF transmitters and receivers may include various DSPcircuitry including at least one power amplifier (PA) and at least onelow-noise amplifier (LNA), respectively. The RF transmitters andreceivers may, in turn, be coupled to one or more antennas. For example,in some implementations, the wireless communication device 500 caninclude, or be coupled with, multiple transmit antennas (each with acorresponding transmit chain) and multiple receive antennas (each with acorresponding receive chain). The symbols output from the modem 502 areprovided to the radio 504, which then transmits the symbols via thecoupled antennas. Similarly, symbols received via the antennas areobtained by the radio 504, which then provides the symbols to the modem502.

The processor 506 can include an intelligent hardware block or devicesuch as, for example, a processing core, a processing block, a centralprocessing unit (CPU), a microprocessor, a microcontroller, a digitalsignal processor (DSP), an application-specific integrated circuit(ASIC), a programmable logic device (PLD) such as a field programmablegate array (FPGA), discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. The processor 506 processes information receivedthrough the radio 504 and the modem 502, and processes information to beoutput through the modem 502 and the radio 504 for transmission throughthe wireless medium. For example, the processor 506 may implement acontrol plane and MAC layer configured to perform various operationsrelated to the generation and transmission of MPDUs, frames or packets.The MAC layer is configured to perform or facilitate the coding anddecoding of frames, spatial multiplexing, space-time block coding(STBC), beamforming, and OFDMA resource allocation, among otheroperations or techniques. In some implementations, the processor 506 maygenerally control the modem 502 to cause the modem to perform variousoperations described above.

The memory 504 can include tangible storage media such as random-accessmemory (RAM) or read-only memory (ROM), or combinations thereof. Thememory 504 also can store non-transitory processor- orcomputer-executable software (SW) code containing instructions that,when executed by the processor 506, cause the processor to performvarious operations described herein for wireless communication,including the generation, transmission, reception and interpretation ofMPDUs, frames or packets. For example, various functions of componentsdisclosed herein, or various blocks or steps of a method, operation,process or algorithm disclosed herein, can be implemented as one or moremodules of one or more computer programs.

FIG. 6A shows a block diagram of an example AP 602. For example, the AP602 can be an example implementation of the AP 102 described withreference to FIG. 1 . The AP 602 includes a wireless communicationdevice (WCD) 610 (although the AP 602 may itself also be referred togenerally as a wireless communication device as used herein). Forexample, the wireless communication device 610 may be an exampleimplementation of the wireless communication device 500 described withreference to FIG. 5 . The AP 602 also includes multiple antennas 620coupled with the wireless communication device 610 to transmit andreceive wireless communications. In some implementations, the AP 602additionally includes an application processor 630 coupled with thewireless communication device 610, and a memory 640 coupled with theapplication processor 630. The AP 602 further includes at least oneexternal network interface 650 that enables the AP 602 to communicatewith a core network or backhaul network to gain access to externalnetworks including the Internet. For example, the external networkinterface 650 may include one or both of a wired (for example, Ethernet)network interface and a wireless network interface (such as a WWANinterface). Ones of the aforementioned components can communicate withother ones of the components directly or indirectly, over at least onebus. The AP 602 further includes a housing that encompasses the wirelesscommunication device 610, the application processor 630, the memory 640,and at least portions of the antennas 620 and external network interface650.

FIG. 6B shows a block diagram of an example STA 604. For example, theSTA 604 can be an example implementation of the STA 104 described withreference to FIG. 1 . The STA 604 includes a wireless communicationdevice 615 (although the STA 604 may itself also be referred togenerally as a wireless communication device as used herein). Forexample, the wireless communication device 615 may be an exampleimplementation of the wireless communication device 500 described withreference to FIG. 5 . The STA 604 also includes one or more antennas 625coupled with the wireless communication device 615 to transmit andreceive wireless communications. The STA 604 additionally includes anapplication processor 635 coupled with the wireless communication device615, and a memory 645 coupled with the application processor 635. Insome implementations, the STA 604 further includes a user interface (UI)655 (such as a touchscreen or keypad) and a display 665, which may beintegrated with the UI 655 to form a touchscreen display. In someimplementations, the STA 604 may further include one or more sensors 675such as, for example, one or more inertial sensors, accelerometers,temperature sensors, pressure sensors, or altitude sensors. Ones of theaforementioned components can communicate with other ones of thecomponents directly or indirectly, over at least one bus. The STA 604further includes a housing that encompasses the wireless communicationdevice 615, the application processor 635, the memory 645, and at leastportions of the antennas 625, UI 655, and display 665.

Aspects of transmissions may vary based on a distance between atransmitter (for example, an AP 102 or a STA 104) and a receiver (forexample, another AP 102 or STA 104). Wireless communication devices maygenerally benefit from having information regarding the location orproximities of the various STAs 104 within the coverage area. In someexamples, relevant distances may be computed using ranging proceduresbased on round-trip time (RTT). Additionally, in some implementations,APs 102 and STAs 104 may be configured to perform ranging operations.Each ranging operation may involve an exchange of fine timingmeasurement (FTM) frames (such as those defined in the IEEE 802.11mcspecification or revisions or updates thereof). FIG. 7 shows a timingdiagram illustrating an example process for performing a rangingoperation 700. The process for the ranging operation 700 may beconjunctively performed by two wireless devices 702 a and 702 b, whichmay each be an example of an AP 102 or a STA 104.

The ranging operation 700 begins with the first wireless device 702 atransmitting an initial FTM range request frame 704 at time t_(0,1).Responsive to successfully receiving the FTM range request frame 704 attime t_(0,2), the second wireless device 702 b responds by transmittinga first ACK 706 at time t_(0,3), which the first wireless device 702 areceives at time t_(0,4). The first wireless device 702 a and the secondwireless device 702 b then exchange one or more FTM bursts, which mayeach include multiple exchanges of FTM action frames (hereinafter simply“FTM frames”) and corresponding ACKs. One or more of the FTM requestframe 704 and the FTM action frames (hereinafter simply “FTM frames”)may include FTM parameters specifying various characteristics of theranging operation 700.

In the example shown in FIG. 7 , in a first exchange, beginning at timet_(1,1), the second wireless device 702 b transmits a first FTM frame708. The second wireless device 702 b records the time t_(1,1) as thetime of departure (TOD) of the first FTM frame 708. The first wirelessdevice 702 a receives the first FTM frame 708 at time t_(1,2) andtransmits a first acknowledgement frame (ACK) 710 to the second wirelessdevice 702 b at time t_(1,3). The first wireless device 702 a recordsthe time t_(1,2) as the time of arrival (TOA) of the first FTM frame708, and the time t_(1,3) as the TOD of the first ACK 710. The secondwireless device 702 b receives the first ACK 710 at time t_(1,4) andrecords the time t_(1,4) as the TOA of the first ACK 710.

Similarly, in a second exchange, beginning at time t_(2,1), the secondwireless device 702 b transmits a second FTM frame 712. The second FTMframe 712 includes a first field indicating the TOD of the first FTMframe 708 and a second field indicating the TOA of the first ACK 710.The first wireless device 702 a receives the second FTM frame 712 attime t_(2,2) and transmits a second ACK 714 to the second wirelessdevice 702 b at time t_(2,3). The second wireless device 702 b receivesthe second ACK 714 at time t_(2,4). Similarly, in a third exchange,beginning at time t_(3,1), the second wireless device 702 b transmits athird FTM frame 716. The third FTM frame 716 includes a first fieldindicating the TOD of the second FTM frame 712 and a second fieldindicating the TOA of the second ACK 714. The first wireless device 702a receives the third FTM frame 716 at time t_(3,2) and transmits a thirdACK 718 to the second wireless device 702 b at time t_(3,3). The secondwireless device 702 b receives the third ACK 718 at time t_(3,4).Similarly, in a fourth exchange, beginning at time t_(4,1), the secondwireless device 702 b transmits a fourth FTM frame 720. The fourth FTMframe 720 includes a first field indicating the TOD of the third FTMframe 716 and a second field indicating the TOA of the third ACK 718.The first wireless device 702 a receives the fourth FTM frame 720 attime t_(4,2) and transmits a fourth ACK 722 to the second wirelessdevice 702 b at time t_(4,3). The second wireless device 702 b receivesthe fourth ACK 722 at time t_(4,4).

The first wireless device 702 a determines a range indication based onthe TODs and TOAs described above. For example, in implementations orinstances in which an FTM burst includes four exchanges of FTM frames asdescribed above, the first wireless device 702 a may be configured todetermine a round trip time (RTT) between itself and the second wirelessdevice 702 b based on Equation 1 below.

$\begin{matrix}{{RTT} = {{\frac{1}{3}\left( {{\sum_{k = 1}^{3}t_{4,k}} - {\sum_{k = 1}^{3}t_{1,k}}} \right)} - \left( {{\sum_{k = 1}^{3}t_{3,k}} - {\sum_{k = 1}^{3}t_{2,k}}} \right)}} & (1)\end{matrix}$

In some implementations, the range indication is the RTT. Additionally,or alternatively, in some implementations, the first wireless device 702a may determine an actual approximate distance between itself and thesecond wireless device 702 b, for example, by multiplying the RTT by anapproximate speed of light in the wireless medium. In such instances,the range indication may additionally or alternatively include thedistance value. Additionally, or alternatively, the range indication mayinclude an indication as to whether the second wireless device 702 b iswithin a proximity (for example, a service discovery threshold) of thefirst wireless device 702 a based on the RTT. In some implementations,the first wireless device 702 a may then transmit the range indicationto the second wireless device 702 b, for example, in a range report 724at time t_(5,1), which the second wireless device receives at timet_(5,2).

Ranging operations (such as the ranging operation 700 of FIG. 7 ) may beused in various proximity-based applications such as, for example,unlocking a vehicle with a mobile phone. A wireless communication devicewithin the mobile phone may communicate with a wireless communicationdevice within the vehicle to perform FTM-based ranging operations suchas described with respect to FIG. 7 . For example, the mobile phone mayindicate its distance to the vehicle by transmitting FTM frames to thevehicle and providing feedback (ACKs) regarding FTM frames received fromthe vehicle. Similarly, the vehicle may determine its distance to themobile phone by transmitting FTM frames to the mobile phone andproviding feedback (ACKs) regarding FTM frames received from the mobilephone. The vehicle may unlock its doors (or other compartments) if itdetermines that the mobile phone is within a threshold proximity of thevehicle.

As described with reference to FIG. 7 , distance calculations are basedon TOAs and TODs of PPDUs (such as FTM frames and ACKs) exchangedbetween a first wireless communication device and a second wirelesscommunication device. In some implementations, a wireless communicationdevice may determine the TOA of an incoming PPDU based, at least inpart, on the time at which the wireless communication device completesreception of an LTF field of the PPDU. The LTF field includes a number(L) of LTF sequences modulated on a number (N) of subcarriers. FIG. 8Ashows a frequency diagram of an example LTF sequence 800 usable forcommunications between wireless communication devices. The LTF sequence800 is a frequency-domain representation of an LTF symbol. As shown inFIG. 8A, a non-zero modulation symbol is modulated on each of the Nsubcarriers associated with the LTF sequence 800. Each modulation symbolmay represent a number or pattern of bit values that depends on the typeof modulation scheme being used. For example, modulation symbols mappedto a binary phase shift keying (BPSK) constellation may each represent asingle bit (0 or 1). Similarly, modulation symbols mapped to aquadrature phase shift keying (QPSK) constellation may each represent atwo-bit pattern (00, 01, 10, or 11). The sequence of modulation symbolsmapped across all N subcarriers is collectively referred to as an “LTFsequence.”

LTF sequences conforming to existing versions of the IEEE 802.11standard are encoded or modulated based on deterministic functions. Inother words, a wireless communication device with knowledge of thefunction used to generate the LTF sequence (such as defined by the IEEE802.11 standards) may observe a portion of an LTF sequence and determineor predict the remainder of the LTF sequence based on the observedportion. FIG. 8B shows a timing diagram of an example LTF symbol 810usable for communications between wireless communication devices. TheLTF symbol 810 may be a time-domain representation of the LTF sequence800 of FIG. 8A. An inverse Fourier transform may be used to map variousmodulation symbols spanning the bandwidth of the frequency-domain LTFsequence 800 to various portions of the time-domain LTF symbol 810. Forexample, a beginning portion 801 of the time-domain LTF symbol 810 maycarry modulation symbols spread throughout the bandwidth of thefrequency-domain LTF sequence 800.

Aspects of the present disclosure recognize that an attacker (orunintended receiving device) may intercept a beginning portion 801 ofthe LTF symbol 810 transmitted by a transmitting device to a receivingdevice. Using a deterministic function, the attacker may determine orpredict the remainder of the LTF sequence based only on informationincluded in the beginning portion 801. The attacker may then transmit acopy of a tail portion 802 of the LTF symbol 810 to the receiving devicebefore the transmitting device has completed its transmission of theoriginal LTF symbol 810. Accordingly, the attacker may trick thereceiving device into thinking the transmitting device is closer than itactually is.

Various aspects relate generally to LTFs used in wirelesscommunications, and more particularly, to generating a secure LTF thatis difficult to decode or replicate by observing only a portion of theLTF. In some aspects, the secure LTF may include a randomized bitsequence that is difficult, if not impossible, to replicate by anydevice other than the transmitting device and the intended receivingdevice (using a secure key previously shared over a secure wirelesslink). For example, the transmitting device may use a block or a streamcipher to generate a pseudorandom bit sequence and may select a subsetof bits of the pseudorandom bit sequence to be mapped to a sequence ofmodulation symbols representing an LTF symbol of the secure LTF. Morespecifically, each of the modulation symbols is mapped to a respectiveone of a number of subcarriers spanning a bandwidth of the secure LTF.The transmitting device may further transmit a PPDU that includes thesecure LTF to the receiving device.

Particular implementations of the subject matter described in thisdisclosure can be implemented to realize one or more of the followingpotential advantages. In some implementations, the described techniquescan be used to improve the security of LTFs used in wirelesscommunications. For example, LTF sequences conforming to existingversions of the IEEE 802.11 standard are encoded or modulated based ondeterministic functions. As a result, an attacker (or unintendedreceiving device) may receive a portion of an LTF sequence and determineor predict the remainder of the LTF sequence based on the receivedportion. A sophisticated attacker may even copy or spoof the LTFsequence before the transmitting device has finished transmitting theoriginal LTF sequence to the receiving device. For example, the attackermay transmit the spoofed LTF sequence to the receiving device to causeerrors in channel or timing measurements by the receiving device. Byrandomizing the modulation symbols associated with individual LTFsequences, aspects of the present disclosure may prevent orsubstantially delay such attacks on LTF sequences long enough to renderthe attacks ineffective.

FIGS. 9A and 9B show block diagrams of an example transmit (TX)processing chain of a wireless communication device according to someimplementations. In some implementations, the TX processing chain may beconfigured to transmit an LTF symbol 914 as part of an LTF field of aPPDU. For example, the PPDU may be an FTM frame. More specifically, FIG.9A shows a first portion 900A of the TX processing chain and FIG. 9Bshows a second portion 900B of the TX processing chain. In someimplementations, the wireless communication device may be an AP such asAPs 102 or 602 of FIGS. 1 and 6 , respectively. In some otherimplementations, the wireless communication device may be a STA such asSTAs 104 or 604 of FIGS. 1 and 6 , respectively. With reference forexample to FIG. 5 , the TX processing chain may include portions of themodem 502 and the radio 504.

With reference to FIG. 9A, the first portion 900A of the TX processingchain includes a pseudorandom generator 910, an LTF sequence selector920, a modulator 930, and a spatial stream (SS) mapper 940. Thepseudorandom generator 910 is configured to generate a pseudorandom bitsequence (PRBS) 902. In some implementations, the pseudorandom generator910 may generate the pseudorandom bit sequence 902 based on an output ofa cipher such as, for example, an advanced encryption standard (AES)block cipher, a hash operation, or a stream cipher. Example suitablestream ciphers may include, but are not limited to, Grain and Grain-128astream ciphers.

In some implementations, the pseudorandom generator 910 may beimplemented in the PHY layer of the wireless communication device. Forexample, aspects of the present disclosure recognize that the controlinterface between the MAC layer and the PHY layer operates at relativelylow speeds. Thus, it may not be feasible to implement the pseudorandomgenerator 910 in the MAC layer, as the interface would create abottleneck in transferring a large pseudorandom bit sequence from theMAC layer to the PHY layer. In some aspects, the pseudorandom generator910 may receive a relatively small number (<300) of secure bits 901 fromthe MAC layer to be used to initialize the cipher block. The secure bits901 may include a key and an initialization vector.

The LTF sequence selector 920 selects a pattern of LTF bits 904corresponding to a subset of the pseudorandom bit sequence 902 and themodulator 930 modulates the LTF bit pattern 904 onto a number (N) ofsubcarriers to produce an LTF sequence 906. As described with referenceto FIG. 8A, the LTF sequence 906 may include a sequence of non-zeromodulation symbols. Each modulation symbol of the LTF sequence 906 mayrepresent a respective subset of bit values of the LTF bit pattern 904.Thus, an overall length (P) of the LTF bit pattern 904 may depend on atype of modulation scheme implemented by the modulator 930 (where P is amultiple of N).

In some implementations, the modulator 930 may implement a quadratureamplitude modulation (QAM) scheme. Higher-order modulation schemes(higher than QPSK) are generally more sensitive to intercarrierinterference (ICI), which tends to degrade the performance of orthogonalfrequency-division multiplexing (OFDM) transmissions. However, aspectsof the present disclosure recognize that the presence of ICI in LTFtransmissions increases the difficulty of decoding or replicating theLTF transmissions. In some aspects, the modulator 930 may implement a16-QAM or higher-order modulation scheme (such as 64-QAM or 256-QAM,among other examples) to balance the advantages (increase decodingdifficulty) with the disadvantages (degrade OFDM performance) of ICI.Since each 16-QAM symbol represents a pattern of four bits, the LTFsequence selector 920 may output an LTF bit pattern 904 of length 4N(P=4*N).

In some other implementations, the modulator 930 may implement a QAMscheme and a phase-shift keying (PSK) scheme. For example, in someaspects, the modulator 930 may include a QAM modulator 932 and a PSKmodulator 934. The QAM modulator 932 may map a first subset of bits ofthe LTF bit pattern 904 to a set of QAM symbols 905(1) according to aQAM modulation technique (such as 64-QAM). The PSK modulator 934 may mapa second subset of bits of the LTF bit pattern 904 to a set of PSKsymbols 905(2) according to PSK modulation techniques (such as 4-PSK).The modulator 930 further combines the QAM symbols 905(1) with the PSKsymbols 905(2) to produce the LTF sequence 906. By generating the LTFsequence 906 based on multiple modulation techniques (such as QAM andPSK), aspects of the present disclosure may further improve the securityof the LTF transmissions. For example, by combining 64-QAM with 4-PSKmodulation techniques, the resulting LTF sequence 906 may be asdifficult to decode as a 256-QAM sequence while the ranging performanceremains substantially the same as a 64-QAM sequence. As a result, theLTF sequence 906 is difficult, if not impossible, to predict by anydevice (other than the intended receiving device) when observing aportion of the secure LTF.

In some implementations, the LTF sequence selector 920 may select theLTF bit pattern 904 from a portion of the pseudorandom bit sequence 902.As described above, deterministic bit patterns (such as bit patternswith repetitions) in an LTF sequence may be easily decoded or replicatedby an attacker. Further, the LTF field of a PPDU may include a number(L) of different LTF symbols representing L respective LTF sequences. Toincrease the difficulty of such attacks, the LTF sequence selector 920may ensure that the LTF bit pattern 904 associated with each of the LLTF sequences includes a unique set or sequence of bits from thepseudorandom bit sequence 902.

In some implementations, the LTF sequence selector 920 may furtherselect a different LTF bit pattern 904 for each of the L LTF sequences.Selecting a pseudorandom bit sequence for a particular LTF sequenceincreases the difficulty of decoding or replicating that LTF sequencewithin a single LTF symbol duration. However, a persistent attacker mayeventually decode or replicate the LTF bit pattern 904. If the LTF bitpattern 904 for one of the L LTF sequences is reused for another one ofthe L LTF sequences, the attacker may spoof that subsequent LTFsequence. To increase the difficulty of such attacks, the LTF sequenceselector 920 may ensure that a different LTF bit pattern 904 isselected, from the pseudorandom bit sequence 902, for each of the L LTFsequences.

The spatial stream mapper 940 maps the LTF sequence 906 onto a number(M) of spatial streams SS₁-SS_(M) to produce a spatially-mapped LTFsequence 908. For example, the spatial stream mapper 940 may apply aspatial mapping matrix to the set of N modulation symbols of the LTFsequence 906. As a result of the spatial mapping, each of the Nmodulation symbols of the LTF sequence 906 is replicated on each of theM spatial streams SS₁-SS_(M) (as the spatially-mapped LTF sequence 908).In some implementations, the spatial mapping matrix may be a P matrixsuch as defined, for example, by existing versions of the IEEE 802.11standard.

Referring to FIG. 9B, the second portion 900B of the TX processing chainincludes a non-cyclic (NC) phase rotator 960, M inverse discrete Fouriertransforms (IDFTs) 980(1)-980(M), and a transmitter (TX) 990. In someimplementations, the second portion 900B also may include an ICIinjector 950 that may be configured to add one or more non-zero tones orsubcarriers to the spatially-mapped LTF sequence 908 to produce anICI-injected LTF sequence 908′. As described with reference to FIG. 9A,the presence of ICI in LTF transmissions increases the difficulty ofdecoding or replicating an LTF symbol from only a portion of the LTFsymbol. Aspects of the present disclosure further recognize that ICI maybe “added” or injected into the LTF sequence 908 by replacing one ormore null subcarriers of the LTF sequence 908 with non-zero subcarriers.

FIG. 10A shows a frequency diagram of an example LTF sequence 1000 priorto ICI injection according to some implementations. In the example ofFIG. 10A, the wireless channel is subdivided into negative and positivesubchannels. The negative subchannel (left of center frequency) includesnegative-frequency subcarriers and the positive subchannel (right ofcenter frequency) includes positive-frequency subcarriers. As shown inFIG. 10A, the LTF sequence 1000 includes several null subcarriers withina center bandwidth (BW) 1002 of the frequency band. Due to the presenceof the null subcarriers, the non-zero subcarriers adjacent the centerbandwidth 1002 tend to have less ICI than the other non-zero subcarriersof the LTF sequence 1000. Thus, to increase ICI for the non-zerosubcarriers adjacent the center bandwidth 1002, the ICI injector 950 mayreplace one or more of the null subcarriers within the center bandwidth1002 with non-zero subcarriers.

FIG. 10B shows a frequency diagram of an example LTF sequence 1010 afterICI injection according to some implementations. In someimplementations, the LTF sequence 1010 may be an example of the LTFsequence 1000 after the replacement of one or more null subcarriers withnon-zero subcarriers. In the example of FIG. 10B, the subcarriers at theedges of the center bandwidth 1002 have been replaced with non-zerosubcarriers 1004. In some implementations, the modulation symbols of thenon-zero subcarriers 1004 may be the same for each spatial stream towhich the LTF sequence 1010 is mapped. With reference for example toFIG. 9B, the ICI injector 950 may inject the same modulation symbolsinto the LTF sequence 908 for each of the spatial streams SS₁-SS_(M). Insome other implementations, the modulation symbols of the non-zerosubcarriers 1004 may be different for different spatial streams. Withreference for example to FIG. 9B, the ICI injector 950 may injectdifferent modulation symbols into the LTF sequence for each of thespatial streams SS₁-SS_(M).

The non-cyclic phase rotator 960 is configured to add phase rotations oroffsets to the various spatial streams SS₁-SS_(M) associated with theLTF sequence 908′ (or LTF sequence 908) to produce a rotated LTFsequence 912. For example, the phase offsets may prevent unintentionalbeamforming at the receiving device. Unintentional beamforming mayresult from constructive (or destructive) interference of multiplespatial streams caused by multipath propagation. In accordance withexisting versions of the IEEE 802.11 standard, cyclic shift diversity(CSD) would be applied to the various spatial streams to offset thephases of each spatial stream and thus avoid unintentional beamformingat the receiving device. However, aspects of the present disclosurerecognize that CSD may not be suitable for secure LTFs. Because thephase rotations are cyclic, an attacker can observe the CSD on one ormore spatial streams and use the knowledge of the CSD to predict thephase offset of the LTF transmitted on another spatial stream.

In some implementations, the non-cyclic phase rotator 960 may applynon-cyclic phase rotations to the modulation symbols modulated on thevarious spatial streams SS₁-SS_(M). As a result, the phase rotationsapplied to one of the spatial streams SS₁-SS_(M) cannot be determined bycyclically delaying or shifting the phase rotations applied to anotherof the spatial streams SS₁-SS_(M). In some aspects, the non-cyclic phaserotator 960 may apply pseudorandom phase rotations across the varioussubcarriers associated with each of the spatial streams SS₁-SS_(M). Morespecifically, the non-cyclic phase rotator 960 may apply a different setof pseudorandom phase rotations to each of the spatial streamsSS₁-SS_(M) (for a total of M sets of pseudorandom phase rotations). Insome other aspects, the non-cyclic phase rotator 960 may apply a number(K) of different phase rotations per spatial stream to the modulationsymbols modulated on various subcarriers associated with the LTFsequence 908′.

In some implementations, the number K of phase rotations may be lessthan the number N of subcarriers associated with the LTF sequence 908′(K<N). As a result, at least one of the K phase rotations may be appliedto two or more modulation symbols modulated on different subcarriers.Aspects of the present disclosure recognize that applying fullypseudorandom phase rotations across all N subcarriers (where K=N)creates diversity in the ICI, which weakens the security of the LTF. Forexample, because each of the L LTF sequences is replicated on each ofthe spatial streams SS₁-SS_(M), an attacker may determine the N phaserotations based on differences in ICI between multiple spatial streams.In contrast, applying the same phase rotation to multiple modulationsymbols on different subcarriers leads to consistent ICI acrossdifferent spatial streams, thereby improving the security of the LTF.

In some implementations, the non-cyclic phase rotator 960 may group theN subcarriers into K subcarrier groups and apply a respective one of theK phase rotations to each of the modulation symbols associated with aparticular group of subcarriers. In other words, the non-cyclic phaserotator 960 may apply the same phase rotation to each modulation symbolmodulated on the subcarriers belonging to the same subcarrier group. Insome aspects, each subcarrier group may correspond to a respectivefrequency sub-band. For example, each subcarrier group may span a rangeof frequencies (such as 20 MHz). In some aspects, the number K ofsubcarrier groups may be fixed or non-variable. Accordingly, thenon-cyclic phase rotator 960 may assign a number (S) of subcarriers toeach subcarrier group based on the total number of subcarrier groups(for example, S=N/K). In some other aspects, the number S of subcarriersper subcarrier group may be fixed or non-variable. Accordingly, thenon-cyclic phase rotator 960 may determine the number K of subcarriergroups based on the total number of subcarriers to be assigned to eachsubcarrier group (for example, K=N/S).

The non-cyclic phase rotator 960 may generate a different set of Kunique phase rotations for each of the spatial streams SS₁-SS_(M), forexample, to produce M*K unique phase rotations. In some implementations,the non-cyclic phase rotator 960 may generate the M sets of K phaserotations based on a pseudorandom function. For example, the M sets of Kphase rotations may be generated based on an output of a linear feedbackshift register (LFSR). To ensure that none of the K phase rotations fora given spatial stream is repeated for the same subcarrier group onanother spatial stream, the non-cyclic phase rotator 960 may select theM*K unique phase rotations from unique portions of the output of theLFSR. The non-cyclic phase rotator 960 may reset the state of the LFSRafter generating the M*K unique phase rotations for a given LTFsequence. This ensures that the same M*K unique phase rotations can bereproduced for the next LTF sequence.

In some other implementations, the non-cyclic phase rotator 960 maygenerate the M sets of phase rotations based on a deterministicfunction. For example, the non-cyclic phase rotator 960 maysystematically derive each of the M*K unique phase rotations. Such asystematic function may ensure that none of the K phase rotations for agiven spatial stream is repeated for the same subcarrier group onanother spatial stream. In some aspects, the K phase rotations may begenerated according to a substantially linear function. For example,each of the K phase rotations may represent a respective phaseassociated with a linear “phase ramp.” The non-cyclic phase rotator 960may apply the linear phase rotations in order of increasing magnitude tomodulation symbols associated with a first range of frequencies and mayapply the linear phase rotations in order of decreasing magnitude tomodulation symbols associated with a second range of frequencies.

FIG. 11A shows a frequency diagram of an example phase ramp 1100according to some implementations. In the example of FIG. 11A, thewireless channel is subdivided into negative and positive subchannels(to the left and right, respectively, of center frequency). The phaseramp 1100 is depicted as a linear curve having a positive slope in thenegative subchannel and a linear curve having a negative slope in thepositive subchannel. In some implementations, each of the K unique phaserotations may represent a respective line along the vertical axis whichintersects the linear curves in the negative and positive subchannels.As shown in FIG. 11A, the k^(th) phase rotation may be applied to thei^(th) subcarrier and the −i^(th) subcarrier. Accordingly, the i^(th)and −i^(th) subcarriers may belong to the same subcarrier group (groupA). The K unique phase rotations may be applied, in ascending order, tothe modulation symbols modulated on negative-frequency subcarriers. Inother words, the magnitudes of the phase rotations increase as thefrequencies of the subcarriers increase. The K unique phase rotationsmay be further applied, in descending order, to the modulation symbolsmodulated on positive-frequency subcarriers. In other words, themagnitudes of the phase rotations decrease as the frequencies of thesubcarriers increase.

FIG. 11B shows a frequency diagram of an example phase ramp 1110according to some implementations. In the example of FIG. 11B, thewireless channel is again subdivided into negative and positivesubchannels. The phase ramp 1110 is depicted as a linear curve having anegative slow in the negative subchannel and a linear curve having apositive slope in the positive subchannel. In some implementations, eachof the K unique phase rotations may represent a respective line alongthe vertical axis which intersects the linear curves in the negative andpositive subchannels. The k^(th) phase rotation may be applied to thej^(th) subcarrier and the −j^(th) subcarrier. Accordingly, the j^(th)and −j^(th) subcarriers may belong to the same subcarrier group (groupB). The K unique phase rotations may be applied, in descending order, tothe modulation symbols modulated on negative-frequency subcarriers. Inother words, the magnitudes of the phase rotations decrease as thefrequencies of the subcarriers increase. The K unique phase rotationsalso may be further applied, in ascending order, to the modulationsymbols modulated on positive-frequency subcarriers. In other words, themagnitudes of the phase rotations increase as the frequencies of thesubcarriers increase.

Referring back to FIG. 9B, the non-cyclic phase rotator 960 may apply aphase ramp (such as the phase ramp 1100 or phase ramp 1110 of FIGS. 11Aand 11B, respectively) to the modulation values of the LTF sequence 908′(or LTF sequence 908). To avoid unintentional beamforming, thenon-cyclic phase rotator 960 may adjust the slopes or offsets of thephase ramps for different spatial streams, thus varying the magnitudesof the K phase rotations applied to each of the spatial streamsSS₁-SS_(M). In some implementations, the non-cyclic phase rotator 960may adjust the slopes of the phase ramps by changing the degree ofincline or decline associated with each slope. For example, thenon-cyclic phase rotator 960 may apply phase ramp 1100 or phase ramp1110 to multiple spatial streams but with varying degrees of slope angle(ϕ). In some instances, the slope angle may be zero (ϕ=0), in which casethe non-cyclic phase rotator 960 effectively does not apply a phase rampon the given spatial stream. In some other implementations, thenon-cyclic phase rotator 960 may adjust the slopes of the phase ramps byinverting the slope or curve. For example, the non-cyclic phase rotator960 may alternately apply phase ramp 1100 and phase ramp 1110 to two ormore spatial streams. Still further, in some implementations, thenon-cyclic phase rotator 960 may adjust the offsets of the phase rampsby adding a fixed phase rotation to each of the K unique phaserotations. For example, the non-cyclic phase rotator 960 may move thephase ramps 1100 or 1110 up or down along the vertical axis when appliedto different spatial streams.

In some implementations, the non-cyclic phase rotator 960 may apply anoptimized set of K unique phase rotations for each of the M spatialstreams SS₁-SS_(M). The optimized phase rotations may be configured tominimize the correlation of the LTF sequence between the spatial streamsSS₁-SS_(M). Example optimized phase rotation matrices θ_(K,M) are shownbelow for LTF sequences mapped to K subcarrier groups and M spatialstreams.

${\theta_{2,8} = \begin{pmatrix}1 & 1 & 1 & 1 & 1 & {- j} & {- 1} & j \\1 & {- j} & {- 1} & j & 1 & 1 & 1 & 1\end{pmatrix}}{\theta_{4,8} = \begin{pmatrix}1 & 1 & 1 & 1 & 1 & {- j} & {- j} & j \\1 & 1 & {- j} & 1 & 1 & {- j} & 1 & j \\1 & {- j} & {- j} & j & 1 & 1 & 1 & 1 \\1 & {- j} & 1 & j & 1 & 1 & {- j} & 1\end{pmatrix}}$

In some implementations, a particular LTF sequence may be repeated (orretransmitted) one or more times in the LTF field of a PPDU. Suchrepetitions are to ensure consistent channel estimations by thereceiving device. For example, noise or interference in the wirelesschannel may affect the LTF sequences received by the receiving device.By including repetitions of one or more LTF sequences, the receivingdevice may check for consistency among the channel estimates associatedwith such LTF sequences. Aspects of the present disclosure recognizethat, in some instances, multiple transmissions of the same LTF sequencecan result in residual beamforming. To avoid residual beamforming, thenon-cyclic phase rotator 960 may apply a different set of K unique phaserotations to each repetition of the same LTF sequence.

In some implementations, the TX processing chain may include a spatialstream (SS) remapper 970. The spatial stream remapper 970 may beimplemented in lieu of, or addition to, the non-cyclic phase rotator960. The spatial stream remapper 970 may be configured to (further)reduce or eliminate unintended beamforming by remapping thespatially-mapped LTF sequence 908, the ICI-injected LTF sequence 908′,or the rotated LTF sequence 912, across the spatial streams SS₁-SS_(M)to produce a remapped LTF sequence 912′. In some implementations, thespatial stream remapper 970 may apply a different unitary matrix(referred to herein as a “Q matrix”) to each subcarrier group associatedwith the received LTF sequence. The Q matrix changes the mapping of themodulation symbols across the spatial streams SS₁-SS_(M) on a per-groupbasis. In some aspects, the spatial stream remapper 970 may randomlyselect the Q matrices to be applied to a particular LTF sequence. Forexample, the spatial stream remapper 970 may randomly select the Qmatrices, from a number of stored Q matrices, based on the pseudorandombit sequence 902.

The IDFTs 980(1)-980(M) convert the LTF sequences on the spatial streamsSS₁-SS_(M), respectively, from the frequency domain to the time domain.For example, each IDFT 980 may produce a respective series oftime-varying samples representative of the LTF sequence (such asillustrated in FIG. 8B). The series of samples output by the IDFTs980(1)-980(M) represents a time-domain LTF symbol 914. The LTF symbol914 is provided to the transmitter 990 for transmission, over a wirelesschannel, to a receiving device. The transmitter 990 may include one ormore power amplifiers to amplify the LTF symbol 914 on each of thespatial streams SS₁-SS_(M) for transmission via at least M transmitantennas.

FIGS. 12A and 12B show block diagrams of a receive (RX) processing chainof a wireless communication device according to some implementations.More specifically, FIG. 12A shows a first portion 1200A of the RXprocessing chain and FIG. 12B shows a second portion 1200B of the RXprocessing chain. In some implementations, the wireless communicationdevice may be an AP such as APs 102 or 602 of FIGS. 1 and 6 ,respectively. In some other implementations, the wireless communicationdevice may be a STA such as STAs 104 or 604 of FIGS. 1 and 6 ,respectively. With reference for example to FIG. 5 , the RX processingchain may include portions of the modem 502 and the radio 504.

With reference to FIG. 12A, the first portion 1300A of the RX processingchain includes a receiver (RX) 1210, a number (M) of discrete Fouriertransforms (DFTs) 1220(1)-1220(M), and a non-cyclic (NC) phase rotator1240. The receiver may receive a time-domain LTF symbol 1202, over awireless channel, from a transmitting device. In some aspects, the LTFsymbol 1202 may be transmitted as part of an LTF field of a PPDU. Forexample, the PPDU may be an FTM frame. In some aspects, the receiver1210 may receive the LTF symbol 1210 on M spatial streams SS₁-SS_(M) viaat least M receive antennas. The receiver 1210 may include one or morelow noise amplifiers (LNAs) to amplify the LTF symbol 1202 on each ofthe spatial streams SS₁-SS_(M). The DFTs 1220(1)-1220(M) convert the LTFsymbol 1202 on the spatial streams SS₁-SS_(M), respectively, from thetime domain to the frequency domain. For example, each DFT 1220 mayproduce a respective sequence of modulation symbols representative ofthe LTF symbol 1202 (such as illustrated in FIG. 8A). The sequence ofmodulation symbols output by the DFTs 1220(1)-1220(M) represents afrequency-domain LTF sequence 1204.

In some implementations, the RX processing chain may include an initialspatial stream (SS) demapper 1230. The initial spatial stream demapper1230 may be configured to reverse or undo a spatial stream mapping (orremapping) performed by the spatial stream remapper 970 of FIG. 9B. Forexample, the spatial stream demapper 1230 may apply a conjugatetranspose of the Q matrix (referred to herein as a “Q^(H) matrix”) toeach subcarrier group associated with the LTF sequence 1204. The Q^(H)matrix reverses the mapping (by the Q matrix) of the modulation symbolsacross the spatial streams SS₁-SS_(M) per subcarrier group. In someaspects, the spatial stream demapper 1230 may randomly select the Q^(H)matrices to be applied to a particular LTF sequence 1204. For example,the spatial stream remapper 970 may randomly select the Q^(H) matrices,from a number of stored Q^(H) matrices, based on a pseudorandom bitsequence 1214. In some implementations, the pseudorandom bit sequence1214 may be identical to a pseudorandom bit sequence used to generatethe LTF sequence 1204 (such as the pseudorandom bit sequence 902 ofFIGS. 9A and 9B).

The non-cyclic phase rotator 1240 is configured to add phase rotationsor offsets to the various spatial streams SS₁-SS_(M) associated with theLTF sequence 1204′ (or LTF sequence 1204) to recover a de-rotated LTFsequence 1206. In some aspects, the non-cyclic phase rotator 1240 may beconfigured to reverse or undo a set of phase rotations added to the LTFsequence 1204′ (or LTF sequence 1204) by the non-cyclic phase rotator960 of FIG. 9B. For example, the non-cyclic phase rotator 1240 also mayapply non-cyclic phase rotations to the modulation symbols modulated onthe various spatial streams SS₁-SS_(M). In some aspects, the non-cyclicphase rotator 1240 may apply pseudorandom phase rotations across thevarious subcarriers associated with each of the spatial streamsSS₁-SS_(M). More specifically, the non-cyclic phase rotator 1240 mayapply a different set of pseudorandom phase rotations to each of thespatial streams SS₁-SS_(M) (for a total of M sets of pseudorandom phaserotations). In some other aspects, the non-cyclic phase rotator 1240 mayapply a number (K) of different phase rotations per spatial stream tothe modulation symbols modulated on various subcarriers associated withthe LTF sequence 1204′.

In some implementations, the non-cyclic phase rotator 1240 may group anumber (N) of subcarriers associated with the LTF sequence 1204′ into Ksubcarrier groups and apply a respective one of the K phase rotations toeach of the modulation symbols associated with a particular subcarriergroup. The non-cyclic phase rotator 960 may apply the same phaserotation to each modulation symbol modulated on the subcarriers withinthe same subcarrier group. In some aspects, the number K of subcarriergroups may be fixed or non-variable. Accordingly, the non-cyclic phaserotator 1240 may assign a number (S) of subcarriers to each subcarriergroup based on the total number of subcarrier groups (for example,S=N/K). In some other aspects, the number S of subcarriers persubcarrier group may be fixed or non-variable. Accordingly, thenon-cyclic phase rotator 1240 may determine the number K of subcarriergroups based on the total number of subcarriers to be assigned to eachsubcarrier group (for example, K=N/S).

The non-cyclic phase rotator 1240 may generate a different set of Kunique phase rotations for each of the spatial streams SS₁-SS_(M), forexample, to produce M*K unique phase rotations. In some implementations,the non-cyclic phase rotator 1240 may generate the M sets of K phaserotations based on a pseudorandom function. For example, the M sets of Kphase rotations may be generated based on an output of an LFSR. In someaspects, the non-cyclic phase rotator 1240 may select the M*K uniquephase rotations from unique portions of the output of the LFSR. Thenon-cyclic phase rotator 1240 may reset the state of the LFSR aftergenerating the M*K unique phase rotations for a given LTF sequence.

In some other implementations, the non-cyclic phase rotator 1240 maygenerate the M sets of phase rotations based on a deterministicfunction. For example, the non-cyclic phase rotator 1240 maysystematically derive each of the M*K unique phase rotations. In someaspects, the K phase rotations may be generated according to asubstantially linear function. For example, each of the K phaserotations may represent a respective phase associated with a linearphase ramp (such as the phase ramps 1100 or 1110 of FIGS. 11A and 11B,respectively). With reference for example to FIGS. 11A and 11B, thenon-cyclic phase rotator 1240 may apply the linear phase rotations inorder of increasing magnitude to modulation symbols associated with afirst range of frequencies and may apply the linear phase rotations inorder of decreasing magnitude to modulation symbols associated with asecond range of frequencies.

The non-cyclic phase rotator 1240 may adjust the slopes or offsets ofthe phase ramps for different spatial streams, thus varying themagnitudes of the K phase rotations applied to each of the spatialstreams SS₁-SS_(M). In some implementations, the non-cyclic phaserotator 1240 may adjust the slopes of the phase ramps by changing thedegree of incline or decline associated with each slope. In some otherimplementations, the non-cyclic phase rotator 1240 may adjust the slopesof the phase ramps by inverting the slope or curve. Still further, insome implementations, the non-cyclic phase rotator 1240 may adjust theoffsets of the phase ramps by adding a fixed phase rotation to each ofthe K unique phase rotations.

The non-cyclic phase rotator 1240 also may detect one or more repeatedLTF sequences in an LTF field of a received PPDU. In someimplementations, the non-cyclic phase rotator 1240 may apply a differentset of K unique phase rotations to each repetition of the same LTFsequence. More specifically, the non-cyclic phase rotator 1240 mayreverse or undo the different sets of K unique phase rotations appliedto each repetition of the same LTF sequence by a non-cyclic phaserotator 1240 used to transmit the LTF sequences.

In some implementations, the RX processing chain may include anintercarrier interference (ICI) subtractor 1250 that may be configuredto remove or undo one or more non-zero subcarriers added to the LTFsequence 1206 by the ICI injector 950 of FIG. 9B. As described withreference to FIGS. 10A and 10B, the ICI injector 950 may replace one ormore null subcarriers adjacent a center bandwidth of the LTF sequence1206 with non-zero subcarriers (such as the non-zero null subcarriers1004 of FIG. 10B). The additional non-zero subcarriers are to increasethe ICI of the LTF sequence 1206 and do not carry useful information.Thus, the ICI subtractor 1250 may remove these additional non-zerosubcarriers from the LTF sequence 1206 to produce an ICI-adjusted LTFsequence 1206′.

Referring to FIG. 12B, the second portion 1200B of the RX processingchain includes a spatial stream (SS) demapper 1260, a demodulator 1270,an LTF sequence comparator 1280, and a pseudorandom generator 1290. TheSS demapper 1260 may be configured to reverse or undo a spatial streammapping performed by the spatial stream mapper 940 of FIG. 9A. Forexample, the spatial stream demapper 1230 may apply a conjugatetranspose of the P matrix (referred to herein as a “P^(H) matrix”) tothe LTF sequence 1206′ (or LTF sequence 1206) to recover a de-mapped LTFsequence 1208. As a result of the de-mapping, the modulation symbolsreceived on the M spatial streams SS₁-SS_(M) are consolidated into asingle frequency-domain LTF sequence.

The demodulator 1270 demodulates the LTF sequence 1208 to recover an LTFbit pattern 1212. In some aspects, the demodulator 1270 may beconfigured to reverse or undo a modulation performed by the modulator930 of FIG. 9A. As described with reference to FIG. 8A, the LTF sequence1208 may include a sequence of non-zero modulation symbols (the LTFsequence) each representing a respective subset of bit values of the LTFbit pattern 1212. Thus, an overall length (P) of the LTF bit pattern1212 may depend on a type of modulation scheme implemented by thedemodulator 1270 (where P is a multiple of N). In some implementations,the demodulator 1270 may implement a QAM scheme such as, for example,16-QAM or a higher-order modulation scheme (such as 64-QAM or 256-QAM,among other examples). Accordingly, each modulation symbol associatedwith the LTF sequence 1208 may be a 16-QAM, 64-QAM, or 256-QAM symbol,and the demodulator 1270 may output an LTF bit pattern 1212 of length 4N(P=4*N).

In some implementations, the demodulator 1270 may implement a QAM schemeand a PSK scheme. For example, in some aspects, the demodulator 1270 mayinclude a QAM demodulator 1272 and a PSK demodulator 1274. The QAMdemodulator 1272 may undo a modulation performed by the QAM modulator932 of FIG. 9A to recover a first subset of bits 1209. The PSKdemodulator 1274 may undo a modulation performed by the PSK modulator934 of FIG. 9A to recover a second subset of bits 1211. The demodulator1270 further combines the first subset of bits 1209 and the secondsubset of bits 1211 to produce the LTF bit pattern 1212. As describedabove with reference to FIG. 9A, by combining multiple modulationtechniques (such as QAM and PSK) to recover the LTF bit pattern 1212,aspects of the present disclosure may further improve the security ofthe LTF transmissions. More specifically, the LTF bit pattern 1212 maybe difficult, if not impossible, to predict by any device (other thanthe intended receiving device) when observing a portion of the secureLTF.

The LTF sequence comparator 1280 may compare the LTF bit pattern 1212with a pseudorandom bit sequence (PRBS) 1214 to produce a comparisonresult 1216. The pseudorandom bit sequence 1214 may be generated by thepseudorandom generator 1290. In some implementations, the pseudorandomgenerator 1290 may be identical to the pseudorandom generator 910 ofFIG. 9A. Thus, the pseudorandom bit sequence 1214 is also identical tothe pseudorandom bit sequence 902. The pseudorandom generator 1290 maygenerate the pseudorandom bit sequence 1214 based on an output of acipher such as, for example, an AES block cipher, a hash operation, or astream cipher (Grain or Grain-128a). In some implementations, thepseudorandom generator 1290 may be implemented in the PHY layer of thewireless communication device and may receive a relatively small numberof secure bits 1201(including a key and an initialization vector) fromthe MAC layer to be used to initialize the cipher block.

In some implementations, the comparison result 1216 may indicate whetherthe bit pattern 1212 matches a subset of the pseudorandom bit sequence1214. For example, because the pseudorandom bit sequence 1214 isidentical to the pseudorandom bit sequence 920 used by a transmittingdevice to generate the LTF sequence, the LTF bit pattern 1212 shouldmatch at least a subset of the pseudorandom bit sequence 1214. Thus, thecomparison may be used to verify that the LTF sequence was received fromthe transmitting device (or a trusted source). In some otherimplementations, the comparison result 1216 may include a channelestimate associated with the wireless channel over which the LTFsequence is transmitted. Still further, in some implementations, thecomparison result 1216 may indicate a TOA of a PPDU (such as an FTMframe or ACK) received from the transmitting device. For example, theLTF sequence comparator 1280 may record the TOA upon verifying an L′ LTFsequence of the received PPDU (where the LTF field of the PPDU includesL LTF sequences).

FIG. 13A shows a flowchart illustrating an example process 1300 forwireless communication that supports secure LTFs according to someimplementations. In some implementations, the process 1300 may beperformed by a wireless communication device operating as or within anAP such as one of the APs 102 or 602 of FIGS. 1 and 6 , respectively. Insome other implementations, the process 1300 may be performed by awireless communication device operating as or within a STA such as oneof the STAs 104 or 604 of FIGS. 1 and 6 , respectively.

In some implementations, the process 1300 begins in block 1301 withgenerating a pseudorandom bit sequence. In block 1302, the process 1300proceeds with selecting a first subset of bits of the pseudorandom bitsequence based on a number (N) of subcarriers associated with an LTF ofa PPDU, where a number of bits in the first subset of bits is greaterthan N. In some implementations, the pseudorandom bit sequence may begenerated in a PHY layer of the wireless communication device. In someimplementations, the pseudorandom bit sequence may be generated based onan output of an advanced encryption standard (AES) block cipher. In someaspects, the pseudorandom bit sequence may be generated by generating aset of secure bits in a media access control (MAC) layer of the wirelesscommunication device and initializing the block cipher in the PHY layerof the wireless communication device based on the set of secure bitsfrom the MAC layer.

In block 1303, the process 1300 proceeds with mapping values of thefirst subset of bits to a sequence of first modulation symbolsrepresenting a first LTF symbol of the LTF, where each of the firstmodulation symbols is modulated on a respective one of the Nsubcarriers. In some implementations, the mapping may be performed inaccordance with a QAM scheme. In some aspects, each of the firstmodulation symbols may be a 64-QAM symbol. In some implementations, thefirst subset of bits may be selected from a portion of the pseudorandombit sequence that does not include any repetitions.

In some implementations, the process 1300 may proceed in block 1304,with mapping the sequence of first modulation symbols to a number (M) ofspatial streams. In some implementations, the process 1300 may proceedin block 1305 with applying M sets of pseudorandom phase rotations tothe sequence of first modulation symbols mapped to the M spatialstreams, respectively, where each set of the M sets of pseudorandomphase rotations is different than the remaining M−1 sets of pseudorandomphase rotations. In some aspects, the M sets of first phase rotationsmay be generated based on a pseudorandom output of an LFSR. In block1306, the process 1300 proceeds with transmitting the PPDU, includingthe LTF, over the M spatial streams to a receiving device.

FIG. 13B shows a flowchart illustrating an example process 1310 forwireless communication that supports secure LTFs according to someimplementations. In some implementations, the process 1310 may beperformed by a wireless communication device operating as or within anAP such as one of the APs 102 or 602 of FIGS. 1 and 6 , respectively. Insome other implementations, the process 1310 may be performed by awireless communication device operating as or within a STA such as oneof the STAs 104 or 604 of FIGS. 1 and 6 , respectively.

With reference for example to FIG. 13A, the process 1310 may begin, inblock 1311, after the mapping of the sequence of first modulationsymbols in block 1304 of the process 1300 and before the transmitting ofthe PPDU in block 1306. In block 1311, the process 1310 begins byselecting a second subset of bits of the pseudorandom bit sequence,where the second subset of bits is different than the first subset ofbits. In block 1312, the process 1310 proceeds with mapping values ofthe second subset of bits to a sequence of second modulation symbolsrepresenting a second LTF symbol of the LTF, where each of the secondmodulation symbols is modulated on a respective one of the Nsubcarriers. In block 1313, the process 1310 proceeds with mapping thesequence of second modulation symbols to the M spatial streams. In block1314, the process 1310 proceeds with applying the M sets of pseudorandomphase operations to the sequence of second modulation symbols mapped tothe M spatial streams, respectively.

FIG. 13C shows a flowchart illustrating an example process 1320 forwireless communication that supports secure LTFs according to someimplementations. In some implementations, the process 1320 may beperformed by a wireless communication device operating as or within anAP such as one of the APs 102 or 602 of FIGS. 1 and 6 , respectively. Insome other implementations, the process 1320 may be performed by awireless communication device operating as or within a STA such as oneof the STAs 104 or 604 of FIGS. 1 and 6 , respectively.

With reference for example to FIG. 13A, the process 1320 may begin, inblock 1321, after the mapping of the sequence of first modulationsymbols in block 1304 of the process 1300 and before the transmitting ofthe PPDU in block 1306. In block 1321, the process 1320 begins bymapping the values of the first subset of bits to a sequence of secondmodulation symbols representing a second LTF symbol of the LTF, whereeach of the second modulation symbols being modulated on a respectiveone of the N subcarriers. In block 1322, the process 1320 proceeds withmapping the sequence of second modulation symbols to the M spatialstreams. In block 1323, the process 1320 proceeds with applying M setsof second phase rotations to the sequence of second modulation symbolsmapped to the M spatial streams, respectively, where each set of the Msets of second phase rotations is different than the remaining M−1 setsof second phase rotations and different than the M sets of first phaserotations.

FIG. 14A shows a flowchart illustrating an example process 1400 forwireless communication that supports secure LTFs according to someimplementations. In some implementations, the process 1400 may beperformed by a wireless communication device operating as or within anAP such as one of the APs 102 or 602 of FIGS. 1 and 6 , respectively. Insome other implementations, the process 1400 may be performed by awireless communication device operating as or within a STA such as oneof the STAs 104 or 604 of FIGS. 1 and 6 , respectively.

In some implementations, the process 1400 begins in block 1401 withgenerating a pseudorandom bit sequence. In some implementations, thepseudorandom bit sequence may be generated in a PHY layer of thewireless communication device. In some implementations, the pseudorandombit sequence may be generated based on an output of an AES block cipher.In some aspects, the pseudorandom bit sequence may be generated bygenerating a set of secure bits in a MAC layer of the wirelesscommunication device; and initializing the AES block cipher block in thePHY layer of the wireless communication device based on the set ofsecure bits from the MAC layer.

In block 1402, the process 1400 proceeds with receiving a PPDU, over awireless channel, from a transmitting device. In block 1403, the process1400 proceeds with recovering a sequence of first modulation symbolsfrom an LTF of the received PPDU, where the sequence of first modulationsymbols represents a first LTF symbol of the LTF. In someimplementations, the PPDU may be received on a number (M) of spatialstreams and the operation for recovering the sequence of firstmodulation symbols in block 1403 may include, in block 1404, applying Msets of first phase rotations to the M spatial streams, respectively,where each of the M sets of first phase rotations is different than theremaining M−1 sets of first phase rotations. In some aspects, the M setsof first phase rotations may be generated based on a pseudorandom outputof an LFSR.

In block 1405, the process 1400 proceeds with demodulating each of thefirst modulation symbols from a respective one of a number (N) ofsubcarriers associated with the LTF, where the demodulation of the firstmodulation symbols produces a first subset of bits representing thefirst LTF symbol. In some implementations, each of the first modulationsymbols may be demodulated in accordance with a QAM scheme. In someaspects, each of the first modulation symbols may be a 64-QAM symbol. Inblock 1406, the process 1400 proceeds with estimating the wirelesschannel based on the first subset of bits and the pseudorandom bitsequence.

FIG. 14B shows a flowchart illustrating an example process 1410 forwireless communication that supports secure LTFs according to someimplementations. In some implementations, the process 1410 may beperformed by a wireless communication device operating as or within anAP such as one of the APs 102 or 602 of FIGS. 1 and 6 , respectively. Insome other implementations, the process 1410 may be performed by awireless communication device operating as or within a STA such as oneof the STAs 104 or 604 of FIGS. 1 and 6 , respectively.

With reference for example to FIG. 14A, the process 1410 may begin, inblock 1411, after the demodulating of the first modulation symbols inblock 1405 of the process 1400 and before the estimating of the wirelesschannel in block 1406. In block 1411, the process 1410 begins byrecovering a sequence of second modulation symbols from the LTF of thereceived PPDU, where the sequence of second modulation symbolsrepresents a second LTF symbol of the LTF. In block 1412, the process1410 proceeds with demodulating each of the second modulation symbolsfrom a respective one of the N subcarriers, where the demodulation ofthe second modulation symbols produces a second subset of bitsrepresenting the second LTF symbol, and where the wireless channelestimate being based on the first subset of bits, the second subset ofbits, and the pseudorandom bit sequence.

In some implementations, the sequence of second modulation symbols maybe recovered by applying the M sets of first phase rotations to the Mspatial streams, respectively. In some other implementations, thesequence of second modulation symbols may be recovered by applying Msets of first phase rotations to the M spatial streams, respectively,where each set of the M sets of first phase rotations is different thanthe remaining M−1 sets of first phase rotations.

FIG. 15 shows a block diagram of an example wireless communicationdevice 1500 according to some implementations. In some implementations,the wireless communication device 1500 is configured to perform any ofthe processes 1300 or 1310 described above with reference to FIGS. 13Aand 13B, respectively. In some implementations, the wirelesscommunication device 1500 can be an example implementation of thewireless communication device 500 described above with reference to FIG.5 . For example, the wireless communication device 1500 can be a chip,SoC, chipset, package or device that includes at least one processor andat least one modem (for example, a Wi-Fi (IEEE 802.11) modem or acellular modem).

The wireless communication device 1500 includes a reception component1510, a communication manager 1520, and a transmission component 1530.The transmission component may further include a pseudorandom generatorcomponent 1522, an LTF sequence selection component 1524, an LTF symbolmapping component 1526. Portions of one or more of the components1522-1526 may be implemented at least in part in hardware or firmware.In some implementations, at least some of the components 1522, 1524, or1526 are implemented at least in part as software stored in a memory(such as the memory 508). For example, portions of one or more of thecomponents 1522, 1524, and 1526 can be implemented as non-transitoryinstructions (or “code”) executable by a processor (such as theprocessor 506) to perform the functions or operations of the respectivecomponent.

The reception component 1510 is configured to receive RX signals fromanother wireless communication device. In some implementations, the RXsignals may include feedback responsive to one or more PPDUs transmittedby the wireless communication device 1500. The communication manager1520 is configured to generate secure LTFs to be transmitted with thePPDUs. In some implementations, the pseudorandom generator component1522 may generate a pseudorandom bit sequence; the LTF sequenceselection component 1524 may select a subset of bits of the pseudorandombit sequence based on a number (N) of subcarriers associated with an LTFof a PPDU, where the number of bits in the subset of bits is greaterthan N; and the LTF symbol mapping component 1526 may map values of thesubset of bits to a sequence of modulation symbols representing a LTFsymbol of the LTF, where each of the modulation symbols is modulated ona respective one of the N subcarriers. The transmission component 1530is configured to transmit the PPDU, including the LTF to a receivingdevice. For example, the PPDU may be transmitted as TX signals to theother wireless communication device.

FIG. 16 shows a block diagram of an example wireless communicationdevice 1600 according to some implementations. In some implementations,the wireless communication device 1600 is configured to perform any ofthe processes 1400 or 1410 described above with reference to FIGS. 14Aand 14B, respectively. In some implementations, the wirelesscommunication device 1600 can be an example implementation of thewireless communication device 500 described above with reference to FIG.5 . For example, the wireless communication device 1600 can be a chip,SoC, chipset, package or device that includes at least one processor andat least one modem (for example, a Wi-Fi (IEEE 802.11) modem or acellular modem).

The wireless communication device 1600 includes a reception component1610, a communication manager 1620, and a transmission component 1630.The transmission component may further include a pseudorandom generatorcomponent 1622, an LTF symbol recovery component 1624, an LTF sequencedemodulation component 1626, and a channel estimation component 1628.Portions of one or more of the components 1622-1628 may be implementedat least in part in hardware or firmware. In some implementations, atleast some of the components 1622, 1624, 1626, or 1628 are implementedat least in part as software stored in a memory (such as the memory508). For example, portions of one or more of the components 1622, 1624,1626, and 1628 can be implemented as non-transitory instructions or codeexecutable by a processor (such as the processor 506) to perform thefunctions or operations of the respective component.

The reception component 1610 is configured to receive RX signals fromanother wireless communication device. In some implementations, the RXsignals may include a PPDU received over a wireless channel. Thecommunication manager 1620 is configured to detect and verify secureLTFs in the received PPDUs. In some implementations, the pseudorandomgenerator component 1622 may generate a pseudorandom bit sequence; theLTF symbol recovery component 1624 may recover a sequence of modulationsymbols from an LTF of the received PPDU, where the sequence ofmodulation symbols represents a LTF symbol of an LTF of the PPDU; andthe LTF sequence demodulation component 1626 may demodulate each of themodulation symbols from a respective one of a number (N) of subcarriersassociated with the LTF, where the demodulation of the first modulationsymbols produces a first subset of bits representing the first LTFsymbol; and the channel estimation component 1628 may estimate thewireless channel based on the first subset of bits and the pseudorandombit sequence. The transmission component 1630 is configured to TXsignals to the other wireless communication device. In someimplementations, the TX signals may include feedback based at least inpart on the comparison performed by the bit pattern comparison component1628.

Implementation examples are described in the following numbered clauses:

-   -   1. A method for wireless communication by a wireless        communication device, including:    -   generating a pseudorandom bit sequence;    -   selecting a first subset of bits of the pseudorandom bit        sequence based on a number (N) of subcarriers associated with a        long training field (LTF) of a physical (PHY) layer convergence        protocol (PLCP) protocol data unit (PPDU), a number of bits in        the first subset of bits being greater than N;    -   mapping values of the first subset of bits to a sequence of        first modulation symbols representing a first LTF symbol of the        LTF, each of the first modulation symbols being modulated on a        respective one of the N subcarriers; and    -   transmitting the PPDU, including the LTF, to a receiving device.    -   2. The method of clause 1, where the pseudorandom bit sequence        is generated in a PHY layer of the wireless communication        device.    -   3. The method of any of clauses 1 or 2, where the pseudorandom        bit sequence is generated based on an output of an advanced        encryption standard (AES) block cipher.    -   4. The method of any of clauses 1-3, where the generating of the        pseudorandom bit sequence includes:    -   generating a set of secure bits in a media access control (MAC)        layer of the wireless communication device; and    -   initializing the block cipher in the PHY layer of the wireless        communication device based on the set of secure bits from the        MAC layer.    -   5. The method of any of clauses 1-4, where the mapping of the        values of the first subset of bits to the sequence of first        modulation symbols is performed in accordance with a quadrature        amplitude modulation (QAM) scheme.    -   6. The method of any of clauses 1-5, where each of the first        modulation symbols is a 64-QAM symbol.    -   7. The method of any of clauses 1-6, where the first subset of        bits is selected from a portion of the pseudorandom bit sequence        that does not include any repetitions.    -   8. The method of any of clauses 1-7, further including:    -   mapping the sequence of first modulation symbols to a number (M)        of spatial streams; and    -   applying M sets of first phase rotations to the sequence of        first modulation symbols mapped to the M spatial streams,        respectively, each set of the M sets of first phase rotations        being different than the remaining M−1 sets of first phase        rotations.    -   9. The method of any of clauses 1-8, further including:    -   generating the M sets of first phase rotations based on a        pseudorandom output of a linear feedback shift register (LFSR).    -   10. The method of any of clauses 1-9, further including:    -   selecting a second subset of bits of the pseudorandom bit        sequence, the second subset of bits being different than the        first subset of bits;    -   mapping values of the second subset of bits to a sequence of        second modulation symbols representing a second LTF symbol of        the LTF, each of the second modulation symbols being modulated        on a respective one of the N subcarriers;    -   mapping the sequence of second modulation symbols to the M        spatial streams; and    -   applying the M sets of first phase rotations to the sequence of        second modulation symbols mapped to the M spatial streams,        respectively.    -   11. The method of any of clauses 1-10, where the second subset        of bits is selected from a portion of the pseudorandom bit        sequence that does not include any repetitions or bits from the        first subset.    -   12. The method of any of clauses 1-9, further including:    -   mapping the values of the first subset of bits to a sequence of        second modulation symbols representing a second LTF symbol of        the LTF, each of the second modulation symbols being modulated        on a respective one of the N subcarriers;    -   mapping the sequence of second modulation symbols to the M        spatial streams; and    -   applying M sets of second phase rotations to the sequence of        second modulation symbols mapped to the M spatial streams,        respectively, each set of the M sets of second phase rotations        being different than the remaining M−1 sets of second phase        rotations and different than the M sets of first phase        rotations.    -   13. A wireless communication device including:    -   at least one modem;    -   at least one processor communicatively coupled with the at least        one modem; and    -   at least one memory communicatively coupled with the at least        one processor and storing processor-readable code that, when        executed by the at least one processor in conjunction with the        at least one modem, is configured to perform the method of any        one or more of clauses 1-12.    -   14. A method for wireless communication by a wireless        communication device, the method including:    -   generating a pseudorandom bit sequence;    -   receiving a physical (PHY) layer convergence protocol (PLCP)        protocol data unit (PPDU), over a wireless channel, from a        transmitting device;    -   recovering a sequence of first modulation symbols from a long        training field (LTF) of the received PPDU, the sequence of first        modulation symbols representing a first LTF symbol of the LTF;    -   demodulating each of the first modulation symbols from a        respective one of a number (N) of subcarriers associated with        the LTF, the demodulation of the first modulation symbols        producing a first subset of bits representing the first LTF        symbol; and    -   estimating the wireless channel based on the first subset of        bits and the pseudorandom bit sequence.    -   15. The method of clause 14, where the pseudorandom bit sequence        is generated in a PHY layer of the wireless communication        device.    -   16. The method of any of clauses 14 or 15, where the        pseudorandom bit sequence is generated based on an output of an        advanced encryption standard (AES) block cipher.    -   17. The method of any of clauses 14-16, where the generating of        the pseudorandom bit sequence includes:    -   generating a set of secure bits in a media access control (MAC)        layer of the wireless communication device; and    -   initializing the AES block cipher block in the PHY layer of the        wireless communication device based on the set of secure bits        from the MAC layer.    -   18. The method of any of clauses 14-17, where each of the first        modulation symbols is demodulated in accordance with a        quadrature amplitude modulation (QAM) scheme.    -   19. The method of any of clauses 14-18, where each of the first        modulation symbols is a 64-QAM symbol.    -   20. The method of any of clauses 14-19, where the PPDU is        received on a number (M) of spatial streams, the recovering of        the sequence of first modulation symbols including:    -   applying M sets of first phase rotations to the M spatial        streams, respectively, each set of the M sets of first phase        rotations being different than the remaining M−1 sets of first        phase rotations.    -   21. The method of any of clauses 14-20, further including:    -   generating the M sets of first phase rotations based on a        pseudorandom output of a linear feedback shift register (LFSR).    -   22. The method of any of clauses 14-21, further including:    -   recovering a sequence of second modulation symbols from the LTF        of the received PPDU, the sequence of second modulation symbols        representing a second LTF symbol of the LTF; and    -   demodulating each of the second modulation symbols from a        respective one of the N subcarriers, the demodulation of the        second modulation symbols producing a second subset of bits        representing the second LTF symbol, the wireless channel        estimate being based on the first subset of bits, the second        subset of bits, and the pseudorandom bit sequence.    -   23. The method of any of clauses 14-22, where the recovering of        the sequence of second modulation symbols includes:    -   applying the M sets of first phase rotations to the M spatial        streams, respectively.    -   24. The method of any of clauses 14-22, where the recovering of        the sequence of second modulation symbols includes:    -   applying the M sets of second phase rotations to the M spatial        streams, respectively, each of the M sets of second phase        rotations being different than the remaining M−1 sets of second        phase rotations and different than the M sets of first phase        rotations.    -   25. A wireless communication device including:    -   at least one modem;    -   at least one processor communicatively coupled with the at least        one modem; and    -   at least one memory communicatively coupled with the at least        one processor and storing processor-readable code that, when        executed by the at least one processor in conjunction with the        at least one modem, is configured to perform the method of any        one or more of clauses 14-24.

As used herein, a phrase referring to “at least one of” or “one or moreof” a list of items refers to any combination of those items, includingsingle members. For example, “at least one of: a, b, or c” is intendedto cover the possibilities of: a only, b only, c only, a combination ofa and b, a combination of a and c, a combination of b and c, and acombination of a and b and c.

The various illustrative components, logic, logical blocks, modules,circuits, operations and algorithm processes described in connectionwith the implementations disclosed herein may be implemented aselectronic hardware, firmware, software, or combinations of hardware,firmware or software, including the structures disclosed in thisspecification and the structural equivalents thereof. Theinterchangeability of hardware, firmware and software has been describedgenerally, in terms of functionality, and illustrated in the variousillustrative components, blocks, modules, circuits and processesdescribed above. Whether such functionality is implemented in hardware,firmware or software depends upon the particular application and designconstraints imposed on the overall system.

Various modifications to the implementations described in thisdisclosure may be readily apparent to persons having ordinary skill inthe art, and the generic principles defined herein may be applied toother implementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

Additionally, various features that are described in this specificationin the context of separate implementations also can be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation also can beimplemented in multiple implementations separately or in any suitablesubcombination. As such, although features may be described above asacting in particular combinations, and even initially claimed as such,one or more features from a claimed combination can in some cases beexcised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Further, the drawings may schematically depict one or moreexample processes in the form of a flowchart or flow diagram. However,other operations that are not depicted can be incorporated in theexample processes that are schematically illustrated. For example, oneor more additional operations can be performed before, after,simultaneously, or between any of the illustrated operations. In somecircumstances, multitasking and parallel processing may be advantageous.Moreover, the separation of various system components in theimplementations described above should not be understood as requiringsuch separation in all implementations, and it should be understood thatthe described program components and systems can generally be integratedtogether in a single software product or packaged into multiple softwareproducts.

What is claimed is:
 1. A method for wireless communication by a wirelesscommunication device, the method comprising: selecting a subset of bitsof a pseudorandom bit sequence based on a number of subcarriersassociated with a long training field (LTF) of a physical (PHY) layerconvergence protocol (PLCP) protocol data unit (PPDU); mapping values ofthe subset of bits to first modulation symbols representing a first LTFsymbol of the LTF in accordance with a type of modulation scheme, eachof the first modulation symbols being modulated on a respective one ofthe number of subcarriers, and each of the first modulation symbolsbeing represented by a respective subset of values of the subset ofbits; and transmitting the PPDU, including the LTF, to a receivingdevice.
 2. The method of claim 1, wherein the pseudorandom bit sequenceis generated in a physical (PHY) layer of the wireless communicationdevice.
 3. The method of claim 1, wherein the pseudorandom bit sequenceis generated based on an output of an advanced encryption standard (AES)block cipher.
 4. The method of claim 3, wherein generating thepseudorandom bit sequence comprises: generating a set of secure bits ina media access control (MAC) layer of the wireless communication device;and initializing the AES block cipher in a physical (PHY) layer of thewireless communication device based on the set of secure bits from theMAC layer.
 5. The method of claim 1, wherein mapping the values of thesubset of bits to the first modulation symbols is performed inaccordance with a quadrature amplitude modulation (QAM) scheme.
 6. Themethod of claim 5, wherein each of the first modulation symbols is a64-QAM symbol.
 7. The method of claim 1, wherein the subset of bits isselected from a portion of the pseudorandom bit sequence that does notinclude any repetitions.
 8. The method of claim 1, further comprising:applying a spatial mapping matrix to the first modulation symbols to mapthe first modulation symbols on a number of spatial streams; andapplying first pseudorandom phase rotations to the number of spatialstreams, wherein each of the first pseudorandom phase rotations isdifferent than a remainder of the first pseudorandom phase rotations,and wherein a different pseudorandom phase rotation, from the firstpseudorandom phase rotations, is applied to each of the number ofspatial streams.
 9. The method of claim 8, further comprising:generating the first pseudorandom phase rotations based on apseudorandom output of a pseudorandom function.
 10. The method of claim8, further comprising: selecting a second subset of bits of thepseudorandom bit sequence, the second subset of bits being differentthan the subset of bits; mapping values of the second subset of bits tosecond modulation symbols representing a second LTF symbol of the LTF,each of the second modulation symbols being modulated on a respectiveone of the number of subcarriers, and each of the second modulationsymbols being represented by a respective subset of values of the secondsubset of bits; applying the spatial mapping matrix to the secondmodulation symbols to map the second modulation symbols on the number ofspatial streams; and applying the first pseudorandom phase rotations tothe number of spatial streams, respectively.
 11. The method of claim 10,wherein the second subset of bits is selected from a portion of thepseudorandom bit sequence that does not include any repetitions or bitsfrom the subset of bits.
 12. The method of claim 8, further comprising:mapping the values of the subset of bits to second modulation symbolsrepresenting a second LTF symbol of the LTF, each of the secondmodulation symbols being modulated on a respective one of the number ofsubcarriers, and each of the second modulation symbols being representedby a respective subset of values of the subset of bits; applying thespatial mapping matrix to the second modulation symbols to map thesecond modulation symbols on the number of spatial streams; and applyingsecond pseudorandom phase rotations to the number of spatial streams,respectively, each set of the second pseudorandom phase rotations beingdifferent than a remainder of the second pseudorandom phase rotationsand different than the first pseudorandom phase rotations.
 13. Awireless communication device comprising: at least one modem; at leastone processor communicatively coupled with the at least one modem; andat least one memory communicatively coupled with the at least oneprocessor and storing processor-readable code that, when executed by theat least one processor in conjunction with the at least one modem, isconfigured to: select a subset of bits of a pseudorandom bit sequencebased on a number of subcarriers associated with a long training field(LTF) of a physical (PHY) layer convergence protocol (PLCP) protocoldata unit (PPDU); map values of the subset of bits to first modulationsymbols representing a first LTF symbol of the LTF in accordance with atype of modulation scheme, each of the first modulation symbols beingmodulated on a respective one of the number of subcarriers, and each ofthe first modulation symbols being represented by a respective subset ofvalues of the subset of bits; and transmit the PPDU, including the LTF,to a receiving device.
 14. The wireless communication device of claim13, wherein mapping the values of the subset of bits to the firstmodulation symbols is performed in accordance with a quadratureamplitude modulation (QAM) scheme, each of the first modulation symbolsbeing a 64-QAM symbol.
 15. The wireless communication device of claim13, wherein execution of the processor-readable code is furtherconfigured to: apply a spatial mapping matrix to the first modulationsymbols to map the first modulation symbols on a number of spatialstreams; and apply first pseudorandom phase rotations to the number ofspatial streams, wherein each of the first pseudorandom phase rotationsis different than a remainder of the first pseudorandom phase rotations,and wherein a different pseudorandom phase rotation, from the firstpseudorandom phase rotations, is applied to each of the number ofspatial streams.
 16. The wireless communication device of claim 15,wherein execution of the processor-readable code is further configuredto: select a second subset of bits of the pseudorandom bit sequence, thesecond subset of bits being different than the subset of bits; mapvalues of the second subset of bits to second modulation symbolsrepresenting a second LTF symbol of the LTF, each of the secondmodulation symbols being modulated on a respective one of the number ofsubcarriers, and each of the second modulation symbols being representedby a respective subset of values of the second subset of bits; apply thespatial mapping matrix to the second modulation symbols to map thesecond modulation symbols on the number of spatial streams; and applythe first pseudorandom phase rotations to the number of spatial streams,respectively.
 17. The wireless communication device of claim 15, whereinexecution of the processor-readable code is further configured to: mapthe values of the subset of bits to second modulation symbolsrepresenting a second LTF symbol of the LTF, each of the secondmodulation symbols being modulated on a respective one of the number ofsubcarriers and each of the second modulation symbols being representedby a respective subset of values of the subset of bits; apply thespatial mapping matrix to the second modulation symbols to map thesecond modulation symbols on the number of spatial streams; and applysecond pseudorandom phase rotations to the number of spatial streams,respectively, each of the second pseudorandom phase rotations beingdifferent than a remainder of the second pseudorandom phase rotationsand different than the first pseudorandom phase rotations.
 18. A methodfor wireless communication by a wireless communication device, themethod comprising: receiving a physical (PHY) layer convergence protocol(PLCP) protocol data unit (PPDU), over a wireless channel, from atransmitting device; recovering first modulation symbols from a longtraining field (LTF) of the PPDU, the first modulation symbolsrepresenting a first LTF symbol of the LTF; demodulating each of thefirst modulation symbols from a respective one of a number ofsubcarriers associated with the LTF, demodulation of the firstmodulation symbols producing a subset of bits representing the first LTFsymbol, and each of the first modulation symbols being represented by arespective subset of values of the subset of bits; and estimating thewireless channel based on whether the subset of bits matches a portionof a pseudorandom bit sequence.
 19. The method of claim 18, wherein thepseudorandom bit sequence is generated in a physical (PHY) layer of thewireless communication device.
 20. The method of claim 18, wherein thepseudorandom bit sequence is generated based on an output of an advancedencryption standard (AES) block cipher.
 21. The method of claim 20,wherein generating the pseudorandom bit sequence comprises: generating aset of secure bits in a media access control (MAC) layer of the wirelesscommunication device; and initializing the AES block cipher block in aphysical (PHY) layer of the wireless communication device based on theset of secure bits from the MAC layer.
 22. The method of claim 18,wherein each of the first modulation symbols is demodulated inaccordance with a quadrature amplitude modulation (QAM) scheme.
 23. Themethod of claim 22, wherein each of the first modulation symbols is a64-QAM symbol.
 24. The method of claim 18, wherein the PPDU is receivedon a number of spatial streams, the method further comprising: applyingfirst pseudorandom phase rotations to the number of spatial streams,wherein each of the first pseudorandom phase rotations is different thana remainder of the first pseudorandom phase rotations, and wherein adifferent pseudorandom phase rotation, from the first pseudorandom phaserotations, is applied to each of the number of spatial streams.
 25. Themethod of claim 24, further comprising: generating the firstpseudorandom phase rotations based on a pseudorandom output of apseudorandom function.
 26. The method of claim 24, further comprising:recovering second modulation symbols from the LTF of the PPDU, thesecond modulation symbols representing a second LTF symbol of the LTF;and demodulating each of the second modulation symbols from a respectiveone of the number of subcarriers, demodulation of the second modulationsymbols producing a second subset of bits representing the second LTFsymbol, each of the second modulation symbols being represented by arespective subset of values of the second subset of bits, and estimatingthe wireless channel being based on the subset of bits, the secondsubset of bits, and the pseudorandom bit sequence.
 27. The method ofclaim 26, wherein recovering the second modulation symbols comprises:applying the first pseudorandom phase rotations to the number of spatialstreams, respectively.
 28. The method of claim 26, wherein recoveringthe second modulation symbols comprises: applying second pseudorandomphase rotations to the number of spatial streams, respectively, each ofthe second pseudorandom phase rotations being different than a remainderof the second pseudorandom phase rotations and different than the firstpseudorandom phase rotations.
 29. A wireless communication devicecomprising: at least one modem; at least one processor communicativelycoupled with the at least one modem; and at least one memorycommunicatively coupled with the at least one processor and storingprocessor-readable code that, when executed by the at least oneprocessor in conjunction with the at least one modem, is configured to:receive a physical (PHY) layer convergence protocol (PLCP) protocol dataunit (PPDU), over a wireless channel, from a transmitting device;recover first modulation symbols from a long training field (LTF) of thePPDU, the first modulation symbols representing a first LTF symbol ofthe LTF; demodulate each of the first modulation symbols from arespective one of a number of subcarriers associated with the LTF,demodulation of the first modulation symbols producing a subset of bitsrepresenting the first LTF symbol, and each of the first modulationsymbols being represented by a respective subset of values of the subsetof bits; and estimate the wireless channel based on whether the subsetof bits matches a portion of a pseudorandom bit sequence.
 30. Thewireless communication device of claim 29, wherein each of the firstmodulation symbols is demodulated in accordance with a quadratureamplitude modulation (QAM) scheme, each of the first modulation symbolsbeing a 64-QAM symbol.
 31. The wireless communication device of claim29, wherein the PPDU is received on a number of spatial streams, andwherein execution of the processor-readable code is further configuredto: apply first pseudorandom phase rotations to the number of spatialstreams, wherein each of the first pseudorandom phase rotations isdifferent than a remainder of the first pseudorandom phase rotations,and wherein a different pseudorandom phase rotation, from the firstpseudorandom phase rotations, is applied to each of the number ofspatial streams.
 32. The wireless communication device of claim 31,wherein execution of the processor-readable code is further configuredto: recover second modulation symbols from the LTF of the PPDU, thesecond modulation symbols representing a second LTF symbol of the LTF;and demodulate each of the second modulation symbols from a respectiveone of the number of subcarriers, demodulation of the second modulationsymbols producing a second subset of bits representing the second LTFsymbol, each of the second modulation symbols being represented by arespective subset of values of the second subset of bits, and estimatingthe wireless channel being based on the subset of bits, the secondsubset of bits, and the pseudorandom bit sequence.
 33. The wirelesscommunication device of claim 32, wherein recovering the secondmodulation symbols comprises: applying the first pseudorandom phaserotations to the number of spatial streams, respectively.
 34. Thewireless communication device of claim 32, wherein recovering the secondmodulation symbols comprises: applying second pseudorandom phaserotations to the number of spatial streams, respectively, each of thesecond pseudorandom phase rotations being different than a remainder ofthe second pseudorandom phase rotations and different than the firstpseudorandom phase rotations.